I try to feed weaned rats before I move to smalls. Are there any other pet stores near you? Not a Petco or Petsmart. If not, then you can feed adult mice and eventually double up on those each feeding and then go on to small rats. I don't start mine on smalls until they are about 600g. The small rats I get are about 60-80g each.
Im in college and have no form of transportation off campus. I have to take a city bus every time I need to go get food for my snake. So unfortunately Petco is my only option.