I found audio file I recorded some time ago, with sounds made of a rubber balloon. The technique was picking, similar to picking strings, just picking the balloon surface, the sounds were somewhat percussive and somewhat tonal. Pitch could be modified by squeezing it and was rather unpredictable. It has been chopped up and made a Kontakt instrument.
Download package - 10MB, includes samples in wav format, 96kHz / 24 bit and mappings for Kontakt 2.2.4 or newer. Free download.
And here's the discussion at KVRaudio.