I got ghost shrimp from PetSmart. They were very cheap- I THINK it was less than $5 for a dozen. They seem to be pretty common. Unfortunately, I put them in a new tank that had not yet cycled: my nitrites and nitrates got too high pretty quickly and they died. Don't know if my turtle ate any of them- I had expected him to. I hope this helps!
They live in warm waters throughout Southern North America.
Often pet stores buy them from people that have netted them out of a canal the day before.
I've seen them arrive in buckets with crawfish and minnows.
They are sold as food for pet fishes.
Ask for "feeder shrimp".
Here they cost $2 per dozen.