I'm likely to have a lot of trouble finding any because I have to wait until a storm tears up kelp hold-fasts, then I have to find them before the tide leaves them high and dry for too long, then I have to tear the kelp hold-fast apart and find a well camouflaged octopus the size of a dangley earring, without hurting it in the process. Maybe every kelp hold fast is filthy with the things, but I wouldn't bet on my ever finding one.
I'll bet that if you ask that supplier for a local pacific octopus with a head bag no larger than an inch, you'll either get a baby bimac, a lilliput, or the dwarf that D mentioned. He must have some local people in Mexico collecting wild animals. Tell him that the tiny octopus can be found in kelp roots washed up after a storm, or in small shells. If he can get one he can probably get more, and if you end up with one of the other species I mentioned, you might not mind. GPO's don't live in southern california or mexico, so don't worry about that.
BTW. The water temp in San Diego right now is probably about 63 degrees, and gets up to about 70 for a few weeks in the summer, so if you get one of these, you might need a chiller (but I suspect nobody knows). The water is usually a little colder and the temp is more stable at 40 to 90 feet deep where kelp roots are, than it is in the shallow tidepools where you find bimacs.
Cannibalistic? Who knows? Somewhere between nobody and almost nobody has ever kept any of these, so that might make you the Tonmo authority on this species. Maybe some scientist kept them and wrote something down, but I can't find a reference to any such paper with Google.