daphnia culture

You could check out Ebay - I once saw daphnia cultures being sold there. The only thing I do know about daphnia is that they require a lot of aeration and a lot of room - I read that someplace. Don't try to culture daphnia with the fish - they won't survive.

if the green water has had guppies in it i doubt there are any daphania so i don't think they can reproduce

If you want to find some daphnids, look for water-filled ditches or small ponds, preferably but not necessarily fish-free. Take a fine-mesh net such as a brine shrimp net, and sweep it through the water; then wash it off in a container of clear water and watch for daphnids- they are round and move in a distinctive jerky manner. Pick out any bigger bugs such as backswimmers (be careful- some water bugs can deliver a painful bite!). Repeat until you have a few hundred in the container.

Colonies are easy. Just fill a container with dechlorinated tap water, add an airstone and you're ready to put the daphnids in! They will feed on green water, filter gunk, active yeast, and powdered fish food. It's best to keep several colonies going at once in case one crashes or goes through a slow period. Temp and lighting are unimportant; room temp and ambient light are fine. Container size can be as small as 1 liter, but bigger containers are more stable. When doing water changes pour the water through a fine mesh net to recover daphnids.