Elodea/Anacharis is pretty common, but different areas have different species. Egeria densa
is listed as a non-native invasive species here in Oregon, so they sell different varieties that, IMO, are not as good.
Water sprite is less common. I'd look for both at local stores, not chain pet stores. If they don't have them, ask, they may be able to get them. It would at least save you the shipping if it comes in with their regular shipment, and most pet stores get a shipment weekly.
Hornwort is a fairly common aquarium plant.
Water lettuce is often available at stores that specialize in ponds.