Like others have said you need the snakes to take f/t before you can really breed rats for them.
If they absolutely won't take f/t then you need to breed either mice or asf's. If asf's are an option I'd go with that hands down. You can have grow out bins and it won't matter how big they get because you can always feed an adult ball python a single asf. Females can get big enough you might feed two but that's still not that much of an issue. I'd get 1.1 asf and once a litter is weaned I'd move them to grow out tubs.