Buying from the listed prey dealers even with shipping is cheaper then the pet stores.
Now, look for rat breeder, fancier clubs an forums near you (book of face). You'll find small prey breeders that you can drive to an pick up prey cheaper. With back yard breeders you can get the right size without the guessing. Some sell live, most will gas for you an then you can freeze them or feed them.
Free prey, check your local animal shelters. You'll find everything plus dogs an cats.
Good luck!