There are dozens of dwarfs and pygmies, but only a few that make it into the aquarium trade (intentionally). The most common is the large egged O. mercatorus from Florida. Usually this is called O. joubini which is a different, small egged species. O. bocki from the Indo-Pacific occasionally shows up. My favorite is O. wolfi, a short-armed pygmy that very much reminds me of blue-rings, but without the TTX. If you have access to southern California waters, you might also find O. micropyrus, one of the smallest of all octopuses. I currently have one and it amzes me how small it is.
Roy