I just went and collect 50-75 gammarus spp. from a local creek. I just flipped over rocks and scooped them up with an aquarium net. My fish go crazy for them. I have found that the ones in my creek like a little slower water with lots of rocks, and they are not under every rock, there distribution seems patchy. Ones from a stream may or may not be adapted to ponds, but it doesn't take long to collect them (at least from Miller Creek).