The sounds I use today for the iPhone and iPad apps started out as a set of audio samples I developed to use with a Wakker MIDI concertina and the Native Instruments Kontakt software synthesizer running on a laptop. Anyone else experimented with the Native Instruments software samplers or have recommendations for others?
Here is a very comprehensive list of Free VSTs and VSTis that might be worth a look. http://forum.cockos.com/showthread.php?t=52382
If you want to pay for your compression software or whatnot… then I can highly recommend http://www.stillwellaudio.com/?page_id=29