Well, no experience with Rubber boas but I've recently purchased a Kenyan Sand Boa which is somewhat similar. I'm not sure why they aren't more popular as they are easy to care for, docile even when young, stay a very managable size, and can get decently heavy bodied (esp. females). As far as breeders, I'd just check on faunaclassifides and kingsnake forums. I know they had normal KSB's for 30-40$ but Rubbers/Rosy's are generally more. But I think that perhaps those groups of snakes are gaining popularity.