It's hard to find good native plant nurseries that carry a good selection of native plants for mesic(wet) areas... Here are the two best I have found:

http://www.shootingstarnursery.com/catalog/index.php?cPath=2

http://www.prairiemoon.com/store/templat...36fc71983f4db8b

As far as sources for truly submersed aquatic plants that are native, I only know of a couple:

http://keystonehatcheries.com/default.asp?id=152

http://www.pondplants1.com/submerged/submerged_pond_plants.htm