Just did a cursory review of midi sound modules out there for ipad/iphone.

Roland has a IOS version of Sound Canvas.
Korg Module Pro
SampleTank
Ravenscroft

Anyway none of them are expensive, even with in-app purchases.

Any user comments on ones they found and like and/or hate.

I'm mostly after 3 classic sounds (b3, rhodes, acoustic piano). anything else is gravy.