I would decide what you are going to feed him first, rats or mice, by what is readily available. I order my frozen rats online in bulk.
A little story...
A few months back I was at PetSmart getting some dog food when I came across their Ball Python enclosure. There was one lonely Normal BP left. As I looked at her (confirmed she was a she) I noticed she had a bad shed and seemed a little small. The temps and humidity were way wrong (20% and around 75 degrees-enclosure was just inside the front door about 20 feet and in the middle of Winter) in the enclosure. SO I took it upon myself to "rescue" this BP from the wicked PetSmart keepers. When I saw that she was 3 months old, I immediately knew she was in danger if I didn't buy her. So I did, took her home, weighed her and put her in quarantine. She, at only 3 months weighed a measly 100g. She was being fed ONE mouse fuzzy a week at PS. I kept her on the one mouse fuzzy for a couple of weeks, then moved her to two mouse fuzzies a week for about a month. Last month, I switched her to rat fuzzies and she is now onto rat pups...and she is thriving! She is currently 5 months old and weighs a little over 250g. She has come along so well and am glad I was able to rescue her from her PetSmart hell.
Long story short, I do use the 10%-15% rule right now in my two young-ins' and they are thriving very, very well.
You didn't rescue her, you bought her. It will just be replaced by another one that will suffer the same treatment. You probably don't want to hear this, but you're just perpetuating the problem by "rescuing" these snakes from Petco and Petsmart...
Lucifer Sam, Siam cat...
Always sitting by your side,
Always by your side...
That cat's something I can't explain...