Different stores may use different terminology for feeder sizes, they're not standard.

If the guy who sold you the snake produced it, it's highly likely he was feeding live and probably raises his own feeders. Most BP babies start on live rodents, either mouse hoppers or rat pups, and then can get converted to f/t after taking a few meals. I would contact him and ask if he can provide feeders for a few more weeks. Once your baby is comfortable and eating in his new home, then attempt to convert him.

Also, if your husbandry isn't correct that can throw any ball python off food. How are you housing him and what are his hot/cool side temperatures?