They'll only grow too fast if you feed em too much....if you think they're growing too fast on frozen food, just feed them less often.
For the OP, you do need to look for large scale rabbit or chicken breeders who are doing a lot of culling. I'm not a big fan of having private individuals keeping giant snakes -- but if you're going to do it anyway, buying the rejects from a breeding operation is probably the least wasteful way to go.

For the OP, you do need to look for large scale rabbit or chicken breeders who are doing a lot of culling. I'm not a big fan of having private individuals keeping giant snakes -- but if you're going to do it anyway, buying the rejects from a breeding operation is probably the least wasteful way to go.