Hello!
I am new to Ardour and was wondering what are some good starting instrument plugins.
Ardour has some included plugins that are very good. There is the General MIDI plugin, Channel 10 is drums, and the ACE Fluid Synth, which uses SF2 instruments. (not the same as SFZ! ) .
SF2 SoundFonts can be found with Google, and here :
https://musical-artifacts.com/artifacts?formats=sf2&tags=soundfont
Sfizz is good for free SFZ SoundFonts :
https://sfz.tools/sfizz/downloads
SFZ Files : https://sfzinstruments.github.io/
Synths:
https://surge-synthesizer.github.io/
https://asb2m10.github.io/dexed/
https://www.discodsp.com/obxd/
https://thewavewarden.com/pages/odin-2
https://www.spitfireaudio.com/bbc-symphony-orchestra-discover
There are plenty more
Your question is almost impossible to answer, because you give no indication of what sort of instruments you’re interested in.
One place to start, though it’s not very instrument-centric: https://linuxdaw.org/
For unusual sample based instruments: get DecentSampler (gratis, but not libre) and visit pianobook.co.uk (search by format “DecentSampler”)
Recent “big” new linux native synths: Surge, Vital(ium), Helm
All or most of U-he’s plugins are available for Linux (not gratis, not libre)
But really, you’d need to say more about what sort of instruments you’re interested in.