I'm planning on getting my 8 month ball python on rats, but I'm not sure what size to feed him. His girth is around 4 - 4 1/2 inches around. I've been feeding him adult mice, what size rat would be equivalent to that?

Also, do you know of any cheap online stores to buy frozen rats? Most of them are super expensive I guess because of the dry ice, but I really need to buy them online because my local pet stores/reptile stores are always either out of stock/don't have the size I need.