Alex,
where are you located? Your profile doesn't list a location. If you're near R.I., Rumford Pet Center has a nice selection. Call first to see if they have the Lilaeopsis. I actually prefer Ehinodorus tenellus, either the broad leaf or the narrow leaf forms as a foreground. For some reason, I've always found Lilaeopsis to be an algae magnet. Also, check out Aqua-Life in Providence. They get nice plants in, but sell out rapidly, so I'd suggest calling ahead there, too.
Other online sources include:
Aquabotanic
http://www.aquabotanic.com/
Excellent customer service, and the owner, Robert, is very knowledgable, and has been growing, importing and selling aquarium plants for a very long time. He gets some very unusual stuff imported from Asia now and again.
Arizona Aquatic Gardens
http://www.azgardens.com/
I've been pleased with plant orders I've placed with them. They get some of the more unusual stuff in, and are very good to deal with.
Also, Aquatic Plant Depot.
http://aquaticplantdepot.com/ind.html
Their list of "unusual" stuff is "Out" in perpetuity, but for standard plants, the prices are good, shipping quick, and all around a good experience. Do watch out for terrarium plants, which will not survive submerged, as I found a few listed in with the aquatic plants.
This should be more than enough to keep you busy!
-Jane
Oh, one last thought on the Lilaeopsis - I think its an easier plant to propogate (grown emersed) and sell. The E. tenellus is actually really tall when grown emersed (more efficient for commercial nurseries, as its faster growth) and so folks always balk that this 5-6" tall plant they buy will become a good foreground, but once it sheds its emersed grown leaves, and is all 1-2" tall submerged growth, its a lovely foreground plant. Also, once transitioned from emergent to submerged growth, I find the E. tenellus to be much faster growing than the Lilaeopsis (which could be why its less prone to algae in my experience, too).