What's your budget? The Apple JamPacks aren't ultra-high end samples, but they're usable (for some projects) and they're reasonably priced.
Garageband itself isn't exactly the "high-end" solution. Moving (at least) to Logic Express would allow for better productions, etc. However, you should be able to find lots of options if you search for "garageband software instruments" at Google, but here's a few examples:
Free samples: There are tons of free samples available on various sites in the
SoundFont format. You can use
some SoundFonts in Garageband -- others may work correctly, or work at all. Google "using soundfonts in gargageband" for more info. In general, Soundfont samples sound decent but aren't top notch.
Shareware samples: Ben Boldt's Garageband/Logic software
instrument collections are samples of various acoustic instruments. Most were apparently made by converting SoundFont samples, so the quality of the samples aren't ultra-fidelity but they're useable. You should be able to find more shareware collections by Googling...
Commercial samples: Many samples available for virtually anything (if you're willing to pay the price). One could easily end up spending a small fortune to obtain a good sample library along with the high-end computer (or computers) needed to play back everything in realtime at 24-bit, 96k, etc. For example, a 24-bit stereo symphony orchestra library with lots of different articulations might cost $1200 and require a MacPro to use it fully. The sky's the limit...
