I'm guessing the petshops don't sell them because they are not an "exotic" species of snake. If I'm not mistaken, ratsnakes are indigenous to the USA. I imagine that a lot of them are wild-caught and then captive bred.
Over here in Switzerland, we have to pay a fair amount for a Leucistic Texas Ratsnake... because it's "exotic".
I remember when I was 11 and living in California, I wanted a western garter snake, but none of the petstores sold them. Then one day after school one just slithered across my path! I caught him, took him home & begged my mother to let me keep him. That was my first snake, Perseus.
It sounds like you're doing a fabulous job as a snake keeper... you're a natural!

I think snakes are much easier to care for than most other animals... well, when they are healthy, at least.
King Arthur is lucky to have you.

Good luck with him!
Andrea (aka Blackjack)