They are not commercially available to the public(as far as anyone on Tonmo knows), although I think there is a place that raises them to sell to educational institutions, but ONLY to educational institutions. You might be able to get a teacher or college bio professor to order one for you.
I've kept two bimacs, both of which I caught myself in southern California. If you know someone who can catch one (with a fishing license) and is willing to give it to you (it's illegal to sell or trade them), that's probably the only way you could get one.
If it weren't for the fact that I only wanted an octopus if I could catch it myself, I would have bought a Hummelincki. They're reportedly at least as good for pets as a bimac, but a little smaller, and no chiller required - Perfect!