We used to get what we called "glass shrimp" when I worked in an aquarium store. Not sure if they are the same as the grass shrimp, but they are small usually under 1 1/2 inch clear freshwater shrimp. If you are looking for a supplier try aquarium stores. I'm not sure if they can order the amounts that are required for a large pond but they are usually available so ask around at fish stores. It might be possible to propigate them in an aquarium yourself and add them to the pond in a heavily weeded area when their numbers are high. Good luck!