I highly recommend Wildlife Nurseries Inc. as a source for plants that serve two basic functions: ornamental and as foliage/feed for wild game-type birds such as ducks, pheasants, etc. While geared towards providing plants for large areas for animal/bird cover, I ordered from Wildlife Nurseries Inc. plants that I use in my different aquatic settings. They have many different kinds that, overall, are fairly easy to grow in my area of central coastal California. Some of the plants that are listed in their mailing list are also available in the horticultural trade.
In dealing with this company you should have available a couple of good texts on aquatic plants so that you will know what the plants are and if they are suitable to your particular setting. And if you have cold-water aquariums you may want to try the Wild Celery roots which is another name for Eelgrass as well as trying the Potamogeton species that they offer. Their latest Spring 2004 price list has just hit the mails so call for one just to see what they have to offer.