The better LFS's will occasionally carry them, mine even carries cuttlefish from time to time. Octopi should be kept in a Octopus specimen tank alone and the size of the tank depends on the maximum size of the Octopus going in. Most are reef safe so you can keep them in reef tank but I would imagine tha they could do some damage to the polyps with those suckers. They need room! You also have to have a proper lid on your tank (weighted, locked) because all cephalopods are rather intelligent creatures and octopi have a knack for lifting the lid and crawling out of their tanks. Make sure it's properly sealed! Also an Octopos can easily be damaged by overflow draoms or pump intakes, you want to ensure it stays clear of these. The can also fit into any crevice that is as large as it's beak (the only solid part of it's body, so even the slightest lift in the lid and they can escape. I've seen that aquariums will put artificial grass carpeting around the aquarium border to prevent escape because octopi are repelled by the texture.
They don't live more than 1 to 2 years depending on the species. They are also not hardy in captivity.
They are quite fond of clams and crustaceans (crab and shrimp) and can be brought in to take care of mantis shrimp issues. But again, it's not fair to the Ocotpus.
Some octopi are lethally toxic to human, others are mildly poisonous, and others may have dangerous bites.
I'm a huge fan of Cephalopods, and I wish they'd be left in the ocean and only be displayed in public aquariums. They don't belong in the Pet trade.