Currently Being ModeratedMay 9, 2013 1:26 AM (in response to Blastor)
The easiest way to extend the range of software instruments in GarageBand would be to buy and install Mainstage 2 from the AppStore (30 US$) . This will include all JamPack and install them for GargeBand as well - the JamPacks bought separately would coast ~500$, so that is really a bargain.
The JamPacks include wide range of additional strings, horns, and other instruments with many articulations, for example:
Alternately you could look at the soundfonts, see HangTime's post here: