Nelson's, in Katy, Texas.
There are also a few places in town that sell water lilies (the Backyard Gardener, in the Heights, is one), but the selection and quality is limited.
There are plenty of mail-order places, including people right on this site. Hopefully they'll chime in.
We also have a waterlily forum, and a plant exchange forum... check the box in the lower right corner and scroll.....