I have been gathering this list the last couple of days, and thought I would share. I basically grabbed everything that still seems to exist on the Internet. If you see anything missing, please add. Enjoy! 
Best SFZ/SF2 instrument sources Open Source, Free, and Commercial (and related misc) As of June 2023
Amazound Samples
Precision Sound
List of Sampled Instrument Libraries
The Other List of Sampled Instrument Libraries
FreePats
Musical Artifacts
https://musical-artifacts.com/
Signal Experiments
Virtual Playing Orchestra
Karoryfer Samples
Versilian Studios Virtual Instruments (VSCO-Community Edition Chamber Orchestra)
https://vis.versilstudios.com/vsco-community.html
Da Real 110 Sample Library
Orange Tree Samples - Jazz Funk Drum Sample Library
Salamander Drumkit
SM Drums
https://sfzinstruments.github.io/drums/sm_drums
99Sounds
BPB Mini Analogue Collection
No Budget Orchestra Archive (on Bandshed.net)
http://www.bandshed.net/sounds/sfz/
http://www.bandshed.net/sounds/sf2/
Production Voices Free Pianos
Production Voices Piano Bundle
Salamander-grandpiano
IvyAudio - Piano-in-162
IvyAudio Libraries
Bigcat Instruments
K18 Upright Piano
Synthfont Soundfonts
S. Christian Collins
Soundfonts 4U
RK Hive Soundfont Archive
Arachno Soundfont
http://maxime.abbey.free.fr/mirror/arachnosoft/files/soundfonts/arachno-soundfont-10-sf2.zip
GoldMidiSf2
Soni Musicae
Zynthian Soundfont Collection Links
https://wiki.zynthian.org/index.php/Sound_Fonts
Learjeff’s Soundfonts
http://web.archive.org/web/20120328123448/http://www.learjeff.net/sf/sf.html
Personal Copy Soundfonts
SoundBurst
http://www.soundburst.com/technolab.html
http://www.soundburst.com/polisix.html
Breity’s Free Instruments
https://www.angelfire.com/goth/TOX/tti.html
The Jazz Page
FMJ-Software Instruments
Ethan’s Original SoundFonts
http://ethanwiner.com/ewsf2.html
Internet Archive Files for Free-Soundfonts-sf2-2019-04
https://archive.org/download/free-soundfonts-sf2-2019-04
Polyphone Soundfonts
GM Synths and Soundfonts
https://drive.google.com/open?id=1TbLOvr6GwqrxteWH8RD7Z7Q7t8q-vsIe
Solarus Forum Has a good list of soundfonts and SFZ
FluidR3_GM.sf2
Open Source Drumkit
https://github.com/crabacus/the-open-source-drumkit
SFZ Mappings for various sample sets
https://github.com/sfzinstruments/mappings
https://github.com/PeteCrighton/Analogue-Drums
SFZ Instruments Github Repositories
https://github.com/orgs/sfzinstruments/repositories?type=all
Banchetto Musicale Sampling Library
http://www.banchetto-musicale.com/en/downloads.htm
Commodore 64 Synthesizer Sessions DELUXE
https://www.kvraudio.com/kvr-developer-challenge/2012/#dc12-6588
https://bedroomproducersblog.sellfy.store/p/commodore-64-synthesizer-sessions-deluxe/
Modular Samples
https://github.com/publicsamples
3 Pages of Open Source Sample Libraries (with SFZ mappings)
https://github.com/orgs/publicsamples/repositories?type=all
Fairlight Sound Library Database
http://www.nattvard.com/iix/database.php
A little freebie Kontakt instrument. (Also, now in SFZ format)
https://www.kvraudio.com/forum/viewtopic.php?f=42&t=480028
http://www.mediafire.com/file/8kx4ad3lcuw8xvt/evssfz.zip
Wave Alchemy Samples and SFZ mappings
https://www.wavealchemy.co.uk/product/ms-20-mini-drums/
https://www.wavealchemy.co.uk/product-category/sounds/samples/
BedRoomProducersBlog BPB
https://bedroomproducersblog.com/tag/sfz/
https://bedroomproducersblog.com/2020/02/19/free-guitar-samples/
https://bedroomproducersblog.com/2017/02/14/plogue-sforzatron/
https://www.plogue.com/products/sforzando-banks.html
SOUNDBANK HOST: ZAMPLER (Check to confirm that Zampler sound banks are playable by sfz players)
https://vstwarehouse.com/sound-bank-host/zampler/
https://www.producerspot.com/product-category/zampler-sounds
Zampler Soundbanks
https://zamplersounds.sellfy.store/zampler/
SZCZ’s Audio Adventures
http://waveforms.fairlyconfusing.net/2013/09/the-wrench.html
http://waveforms.fairlyconfusing.net/2013/09/the-wrench-part-2-bare-hammer.html
http://waveforms.fairlyconfusing.net/2013/07/technics-m33g-feedback-loop-tone.html
http://waveforms.fairlyconfusing.net/2014/09/terrible-piano.html
Samples From Mars
SampleTekk
https://www.sampletekk.com/grand-pianos&product_id=262
https://www.sampletekk.com/grand-pianos?product_id=224
https://www.sampletekk.com/index.php?route=product/product&product_id=291
https://www.sampletekk.com/grand-pianos?product_id=225
https://www.sampletekk.com/index.php?route=product/product&product_id=223
https://www.sampletekk.com/index.php?route=product/product&product_id=346
https://www.sampletekk.com/grand-pianos&product_id=294
https://www.sampletekk.com/index.php?route=product/product&product_id=255
https://www.sampletekk.com/index.php?route=product/product&product_id=243
https://www.sampletekk.com/index.php?route=product/product&product_id=254
https://www.sampletekk.com/grand-pianos&product_id=235
https://www.sampletekk.com/synthesizers?product_id=193&tracking=544f76a2328d8
LootAudio
https://www.lootaudio.com/category/kontakt-instruments/boyss-sound-e-scapes/guitar-di
http://www.retrosampling.se/electricpiano/Electric%20DigiChrome%20Piano%20SF2%20v1.0.zip
http://freedrumkits.net/sound-fonts/
https://bedroomproducersblog.com/2015/04/08/game-boy-drum-kit/
https://bedroomproducersblog.sellfy.store/p/bpb-game-boy-drum-kit/
Soundfonts 4U
ProducerFeed
https://www.producerfeed.com/2011/12/20-synth-soundfonts.html
https://www.producerfeed.com/2010/03/jr-soundfonts.html
https://www.angelfire.com/goth/TOX/tti.html
https://www.producerfeed.com/2009/11/126-hip-hop-soundfonts.html
https://www.producerfeed.com/2012/09/viral-massacre-kit-free-drum-kit-and.html
https://www.producerfeed.com/2010/02/25-piano-soundfonts.html
https://www.producerfeed.com/2010/02/49-guitar-soundfonts.html
https://www.producerfeed.com/2010/04/56-drum-percussion-soundfonts.html
https://www.producerfeed.com/2010/04/saint-james-orchestral-soundfonts.html
https://www.producerfeed.com/2010/06/29-trombone-sax-flute-soundfonts.html
https://cymatics.fm/blogs/production/soundfonts
MuseScore
https://musescore.org/en/handbook/3/soundfonts-and-sfz-files
ZanderJaz
https://www.zanderjaz.com/downloads/soundfonts/synths/
Internet Archive
https://archive.org/details/sf2soundfonts
https://archive.org/download/free-soundfonts-sf2-2019-04
https://archive.org/details/Soundfonts
https://archive.org/details/sc-55-roland-25-gm-soundfonts
https://archive.org/details/500-soundfonts-full-gm-sets
https://archive.org/download/500-soundfonts-full-gm-sets
Les Productions Zvon
http://lesproductionszvon.com/products.htm
http://lesproductionszvon.com/freesounds.htm
https://www.hedsound.com/2019/10/hedorgan-pipe-organ-soundfont.html
Digital Sound Factory
https://www.digitalsoundfactory.com/
Phils Computer Lab - General Midi and Soundfonts
https://www.philscomputerlab.com/general-midi-and-soundfonts.html
http://soundfonts.darkesword.com/
http://www.guraydere.com/soundfonts/sfdetails.aspx?kat=5
https://beats.codenamehippie.com/samples/SoundFrontz/
Michael Picher
https://shop.michaelpichermusic.com/collection/virtual-instrument-sample-libraries
BigFishAudio - Soundfonts
https://www.bigfishaudio.com/SoundFont-Format.html
Lucid Samples
https://www.lucidsamples.com/free-sample-packs/106-free-multi-samples-soundfonts-sf2.html
https://www.pistonsoft.com/soundfonts/chorium.sf2
Producer Loops - Soundfonts
https://www.producerloops.com/download-soundfonts/
PianoBook
Soundfonts folder
https://ftp.lysator.liu.se/pub/awe32/soundfonts/
Geek Funk Labs - SquishBox Soundfonts Collection
https://geekfunklabs.com/download/squishbox-soundfonts/
PlatinumAudioLab.com - Soundfonts
https://www.platinumaudiolab.com/sample_libraries.php?type=multi&format=Soundfont
Lucid Samples
https://www.lucidsamples.com/60-soundfonts-sf2
Quasar Sounds
https://quasarsounds.com/listing-category/soundfonts-sf2/
Decent Samples (Easy to convert to SFZ)
https://www.decentsamples.com/
SoundIron
https://soundiron.com/search?q=sfz
https://github.com/sfzinstruments
https://rytmenpinne.wordpress.com/sounds-and-such/salamander-drumkit/