My friend got his from another reptile breeder, who got them form another reptile breeder etc...... There are a lot of suppliers who carry many different strains of mice, but some only sell to universities or labs. You'll just have to check to see if they will sell to private individuals. Be prepared to pay a fair amount for them though, they run about $3-4 each plus shipping from most suppliers. I think Ace Animals may sell publicly, but I'm not sure. If you do get them from a supplier, be sure to get the outbred strain, as all the inbred lines have lower production and shorter lifespans.

-Evan