Dan, the crayfish will find their way into your pond so there isn't much to worry about that. If you have a fish population, they will usually keep crayfish numbers down.
As far as sourcing grass shrimp, the only good source I know of it Fattig's Fish Farm. TJ, an active member on here sometimes does group orders in the spring to help lower costs. I have plentiful numbers in my pond, if you want to come down to northern VA, I can net you some for free.
