You should try to get Moinia macrocarpa (a Japanese species). This one is
much easier to culture than daphnia species (at least most of the European).
They require less space, tolerate higher temperature and are easily feed
with a yeast-suspension.
Daphnia are filter-feeders and would probably clean your tanks for "green
water", but they do not (as far as I know) remove algae or blue-green
bacteria growing on plants.
If you can get in touch with any local fish- or aquarium societies, they
usually have cultures of daphnia and would hopefully share with you for
free. At least they should know where you can get them.
Kind regards
Hans M Hanslin, Trondheim, Norway