Mine were hiding near the roots of plants. Some were in the open but they were mostly rolling within the grassy zone in the puddle. And they weren't in every puddle, either. Just the grassy one. If I do succeed in growing them, I can send you some. Heck, I could go there right now and grab you a bunch. I doubt they would cost much to ship, I wouldn't even use more than enough water to just keep them wet. If you feel like dropping a couple of dollars on mystery algae balls.
I did notice some strange stuff while reading up. Sometimes they host diseases, and they apparently are regularly eaten by peruvian peoples even though they contain a neurotoxin.