I think we started breeding our own at 6 snakes, but we still ordered FT online. It's taken months to hold back and build the colony, and I think we've been completely free from the bill for 8 months now.

In this thread, there is a section of how to get one litter a week of rats and a nice rotation schedule going.
http://ball-pythons.net/forums/showthread.php?t=52894

I bet you can find the Natal rats/ African soft furreds around there. They breed like crazy and you'll have more than enough in no time.