Back when I was breeding tropical fish I used to get out catching Daphnia. It's much cheaper and easier than buying it, and also a lot of fun. Culturing it yourself might be good too, but I used to find it took a fair bit of space. Even a large swimming pool wasn't enough to keep me as supplied as I'd have liked. I suppose if you didn't need much you might be able to get away with doing it without a swimming pool. I used to find it took a fair while to get a culture going, but I suppose again if you don't want a whole heap it would be easier and quicker.
Go get yourself a fine net, attach it to a long pole and play in your local ponds and swamps. It'll certainly be easy to catch more than 40 Daphnia!
The world hasn't come to an end yet, there are still ponds and swamps to play in. Not everything has to come from a retailer at a price, and exploring the world is an adventure worth more than the money you'll save.
