If you can't locate a feeder breeder (check with your local herp society if you have one) and have to buy from a pet store go with the rats marked "small". You want them just weaned or shortly thereafter. Keep the females away from the male until they are about 250 grams or about 4 to 5 months of age. During that time you can make sure they are all healthy and in great shape to produce some good feeder litters for you.