What are Soundfonts?
Well, they're simply .wav file samples that have been transformed by a SoundFont editor, such as Vienna, into MIDI-controllable instruments which can be loaded onto your Mac and triggered by GarageBand. They're also referred to as .sf2 files, patches or programs, and they're generally put together in groups known as SoundFont Banks, which can contain definitions of up to 128 instruments and one drum set. For a full description, see Wikipedia | Soundfonts
Where can I find them?
sf2midi.com presents a massive collection: At last count over 3,860 free SoundFonts available!
Tons of SoundsFonts nicely categorized. just shy of 1,100! (Piano, Chromatic Percussion, Organ, Guitar, Bass, Solo Strings, Ensemble, Brass, Reed, Pipe, Synth Lead, Synth Pad, Synth Effects, Ethnic, Percussive, Sound Effects, Drumkits and percussive kits, Collections, Miscellaneous)
Over 220 nicely categorized SoundFonts (Bass, Brass, Guitar, Loops (not loops like GB loops, actual instruments that paly sequences), noise, Orch Hit, Organ, Perc, Piano, Strings, Synth ,Voice, Wind)
The Jazz Page
Muted Trumpet, Trombone, Flute, Trumpet, Brass, Tenor Sax, Acoustic Bass, Contrabass, Harmonica, Soprano Sax, Alto Sax. Guitars: Jazzguitar, Nylon Guitar, Steel Guitar, Clean Guitar, Muted Guitar, Overdrive Guitar, Distortion Guitar, Guitar Harmonics.