That weight sounds awfully small for an 8 month-old... I know snakes grow at different rates, but all of mine were at LEAST 500-600g by that age. What has it been eating in the past, and how often? I think it could handle two small-ish mice by now, and I agree about finding a new source for the mice - Petsmart/Petco grossly overcharges, at least around here (since they don't technically sell "feeders," just pet mice).