Which side of the pond are you on? I know a fantastic place here in the UK:

http://www.aquaticplants.eu.com (aka Greenline)

I've ordered many plants from them before including frogbit.

Frogbit is quite a cool plant but I had to chuck it all out in the end because I got a massive aphid infestation! I think my Amano shrimps were having a munch too but I couldn't prove it because of the damage the aphids did!

DaveC