Hi, I'd like to be able to use either my macbook pro with an Akai LPK25 keyboard or ipad to play soundfont files live for sound effects / percussion instruments for theatre use. I don't want to use loops or pre-made music but I would like to be able to easily change sound fonts quickly on the fly. Can anyone suggest the best ( and preferably cheapest) software to do this please? Thanks.

Vsamp is a venerable shareware alternative.
you'll find other candidates at Hit Squad:
http://www.hitsquad.com/smm/mac/SOFTWARE_SAMPLERS/

or with a google search.

since you're working in theater, i hope you're aware of the excellent free version of QLab, the SFX playback software.
good luck.

Thanks Houstonmusic, I'll check those out...