The best time to find big normal females is early in the year, around April or May. Most breeders put their normals up for sale right after laying eggs in the early spring, because the females will be replaced by morph girls.
The second best time is in September and October. Then people looking to make some money for that morph male they suddenly decided they have to have for the breeding season will let a few normal females go, knowing they can get the highest prices because people just getting started are desperate to get their hands on a breeding size female.
The best place to find these girls are on Fauna and Kingsnake classifieds. After that stalk the reptile shows. Craigslist is a long shot at best, the decent females are few and far between, usually mis-sexed, and half of the ones you do find that actually ARE females are neglected and sick and not in any condition to have eggs.
Gale