I've gotten some from William Tricker's.

I got mine from for half price from Van Ness Water Gardens because it had lost its identification. It's never bloomed, but I get a LOT of good tubers for stirfry! I pull them out in the autumn and replant 1-2 tubers into the 18 gallon tubs I raise them in.

Here are sites that I considered for my purchase this past winter:

Ten Mile Creek Nursery, Texas Water Lilies, Perry's Water Garden, Springdale Water Gardens, Pond MegaStore, and BonniesPlants.

Ten Mile Creek has the best variety (127 types), followed by Pond MegaStore (60 or so). I went with Texas Water Lilies because a variety specific to them appealed to me. I was very satisfied with them but have no personal experience with any of the others.