You might also want to give local greenhouses that have dedicated pond sections a try as I personally have had a lot of success finding really cheap water plants at such a place. All the plants in my coldwater black moor tank are from greenhouses and about 1/3 of my tropical ones are. I couldn't find floating plants at any of my lfs' and they have oodles at greenhouses. My fish gobble up duckweed and thus stay away from my more expensive plants. haha