there is or are pygmy whites and from memory it is tetragonna alba, however there is frequent confusion over the name and my first pygmy white isnt a pygmy. I am trying a second from Paul Bromfields but dont remember the name at the moment.
You might also like to look at "helvola" which is a yellow. If you would accept other colours then look at rubra (a pinkish red) and maybe Laydekeri_fulgens also a pinky red what I was sold as that is a pygmy but I am not sure if it is correct. Aurora but that hasnt bloomed for me yet.

If you are in the states you will probably have a greater choice range but the above are available in the UK. Aside from the local garden centre if you are UK have a look at
http://www.pondplants.co.uk/
http://www.bromfieldaquatics.co.uk/
and perhaps http://www.lilieswatergardens.co.uk/...e.asp?cat=A-6B