Where do you live? Do you have any areas of standing water near you that have no fish in them? It doesn't have to be permanent water, as daphnia will be in places that only have water in the spring. If waterfowl visit these waters they will almost surely have daphnia.
I would add that daphnia tolerate very low temps. My culture, which is on the winter swimming pool cover, was going before the ice was melted. About 90% of the surface was still ice covered, and the daphmia were already in full swing as visible when viewing the open water.