Hi Matt,
You can search in this forum for "sound library", "sound libraries", "soundfont" among other terms to find out how many users are getting more sounds.
Some of these you can play directly using your keyboard (eg. Garritan Libraries
www.garritan.com) Others are used solely as "playback devices" by programs such as our own Notation Musician (
www.notation.com/NotationMusician.php) and Notation Composer (
www.notation.com/NotationComposer.php) Many can do both.
If you've got some specific goals in mind (eg. creating compositions for playback or recording, or just experimenting with sounds for live performance) you may get some better specific feedback from other users here.
ttfn,
Sherry