I order f/t online (if I want to feed frozen, it's my only option since the only local reptile store only sells live. I haven't even checked with my local petsmart and petco because I've heard that they can be ridiculously expensive). My ball python has always been on rats. She has been wavering between 135-145 grams and is probably big enough for the rat pups (I actually think she is in between sizes, tbh, but all the fuzzies they sent me in this batch were quite small, only 10-15 g each), but I ordered so many rat fuzzies and, like you said, they can be expensive if you don't order a lot. So I've actually been feeding her 2 fuzzies each feeding (I try to pick two that, together, weigh about 10% her body weight). It's kind of a pain, because I have issues with the 2nd cooling too fast while I wait for her to eat the first. I get around that by keeping the pot of hot water on hand so I can dunk the second rat in it just before feeding it to her so it's warm enough for her to want to eat. Maybe, for the time being, you could buy two mice that together weigh the 10-15% of his body weight and feed those until he is ready to move up to the rats you plan on buying a lot of. Or do what I do and go ahead, order the rat pups, and when he "outgrows" them, just feed him two of the rat pups. It's a little more work but I don't think feeding two will cost you that much more, since rats get a little more expensive each size you go up. In the end, it will save you money since you won't have to buy the individual mice/rats each week, and you won't have leftover wasted rats.