Since you want to feed your feeders to both your ferrets and the ball python that you anticipate getting, you might want to consider looking into African soft-furred rats (ASFs).
I'm no expert, but, from a size standpoint, the ASFs will never be too large to feed to an adult ball python (weigh about 60g as an adult) and are equivalent to a small rat as an adult. The only downside is that they can be a little fussier about the temperatures for breeding (fine with heat, but dislike the cold) and take longer to reach adult size than rats.
I'll look into them. I don't think I'll find any sources in my state(CA), but I'll try!
Thanks for the tip.