There was a previous discussion - scroll down to Nov 6.

It is a combination of laws in certain states where they occur, low rate of production, the snakes not suited to hot weather, and probably a reduced interest compared to more easily obtained CB small boids.

Example: Where I live, I can have some, but can never sell any. I can give them away within the state, but I would never advertise on a classified ad for that purpose.

Luckily, there are breeders that live in states where rubber boas can be advertised w/o issue. They do advertise occasionally. Keep looking on the classifieds. In fact, many here that breed and sell a variety of rosyboas will often have rubber boas too. But again, their production is probably extremely limited.
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Mark