Don't spend all your time reading! Go out and look for local ditches and puddles. Often the spring is good for finding aquatic plants because the ditches and puddles get covered over with algae and emergent plants later in the summer, and the submerged ones are hard to see. Sometimes the small puddles and ditches are better than bigger streams and ponds because they do not have ducks, turtles, and crayfish that eat up plants. Also the small soil seeps that lead into puddles are often high in iron, and that seems to encourage many species These seeps may be good for lots of other nutrients that may be much more dilute in larger ponds and rivers.
Also, remember---It does not have to be actually in the water to be a possible candidate for the aquarium. For example, I have never seen wild Samolus actually under water, or ever very near water, but it is known to be a pretty good aquarium plant. Just as you can not tell, looking at a caterpillar, that it can become a butterfly, you can not tell, looking at an emersed plant whether or not it has the genetic constitution to adapt to living under water. You never know, until you try.
I observed a little, almost dry ditch and spotted some marsilea and perhaps some other plants. Last year, this ditch had a crudload of what looked like anacharis, but it dried up before I could get to it! It was kinda sad seeing lumps big enough to fill a 5 gallon bucket dried up in the sun.
How would you collect the marsilea. I've tried on a few, and I usually pull up leaves. Any suggestions?
Marcelia could be interesting. I think I have heard that there is an American Marcelia species. For getting them up, I would get a trowel or shovel and dig them up. The rhizome is probably deep and well anchored, and that is why you pull up leaves. Perhaps you can get the rhizomes out with your hands if the mud is soft.