I have a lot of water wisteria, r. rotundifolia, and myriophellum. The wisteria can be in small or large pieces up to a couple of feet long, depending on preference. Most of the rotundifolia are clippings 2-6 inches. There are also duckweed, pennywort, and dwarf sagittaria available in smaller quantities. For $20 (local pickup) you get more plants than you can possibly need of the three main species, at least enough to fill a ten gallon fish tank.
All of the plants come from tanks with pest snails and duckweed, so make sure to isolate them for a bit if you care about that.
I have plenty of mystery snails, so take some free with the plants if you want them.
For $10, you can also get a ‘mother plant’ for water wisteria or rotundifolia that comes with a complete root system. The rotundifolia is a sort of carpet 5 inches across with ~20 stems, the wisteria is at least 16 inches tall, some up to three feet.