Where are you located? Have you searched to see if there are any reptile breeders that are somewhat local to you that are willing to help you out (many breed their own prey)?
Petco, as far as I know, doesn't get their rodents locally - they have a contract with huge breeders and they are shipped to them regularly.
Call back some of the smaller pet stores and see if any will tell you where they get their mice from or ask them if they can request that their rodent supplier delivers some hoppers on their next delivery.