Another option is to buy daphnia 'eggs'. They're easy to ship and won't die on you.
You get better hatch rates if you mix in distilled/RO water. Once they hatch, you'll have daphnia for life. Even if you population crash, just leave it alone for a few weeks and they'll come back. You can see white specs on the side of your tank/container. Those are the daphnia 'eggs'.