Some of our bimac owners are buying live food from seafood markets.
I buy from an upscale seafood market because that's the only place nearby where I can get very fresh scallops that have nothing sprayed on them or added. You might have to ask the place you are buying from about each type of seafood, and do tell them that it's for your octopus!
If you expect to order a small bimac, you may find that it cannot deal with a clam at first. Mine still does not realize that a clam is food, a month after his arrival here. A blue crab is very large, too. Maybe you should have a back-up plan using something like pieces of fresh scallops for food until you find out what your octo's food preferences are. If you can get small crabs, that might be good, too, or try some of the small live shrimp.
Please keep us posted about your bimac!