You can try a pet store, but they are really hard to find, and when you do find them they run out a lot. Your best bet is to order frozen off the net. Or just feed live mice.

We have 4 pet stores in topeka. Only two of them sell feeder rats at all ages, and they are almost constantly out. I'm waiting on my first shipment of frozen, and just picked up a pair of rats for breeding. Until then if I can't find the right size rat, I feed mice.