Hi all,
I'd try a £5 for 10 plants on Ebay, I think if you get in the spring people may want it for ponds etc. as well as aquariums.
It is a very good "floater" if you breed Gourami, Betta etc.
I'm not sure there is a "Dwarf Water Lettuce" as such, I think it's size is related to the amount of light and nutrients it receives, the plants in the tank at the moment are about as big as 50p piece, but there clonal "parents" were as big as a Cos lettuce, and had roots several feet long (they came from our landfill leachate remediation work, and had been grown under high pressure Sodium grow-lights in a warm glasshouse). Last year on the pond the same plant managed "tennis ball" sized rosettes.
cheers Darrel