Hoppers are too young to breed. If you really want to raise them, get ahold of some just weaned females, get them to breeding age (10-12 weeks old) and then move them into the breeding enclosure with a young male.

Yes, most chain petstores (petsmart, Petco, petland) will only offer one sex or another. They say its to avoid breeding in the stores. Anyway, unless the mice in stores look good and small, I wouldn't try getting breeders there: most mice in petstores are already adults or even retired breeders from their mass-rodent-producing suppliers.

The best place to go for some good breeding mouse stock would probably be a place that sells mice as feeders for reptiles. They should already have their mice separated by age (or at least size) and they are MUCH cheaper. Or, if you're lucky enough to know someone who breeds mice near you, you could ask them for a few youngsters to start your own operation.

Sorry that was so long...