Most collectors don't know anything about the different species and you're lucky to even get a confirmation of where the octopus came from, which can narrow down the kind of species you would get. To them it's all about the paycheck and "an octopus is an octopus".
Bimacs are fine in a 50-55 gallon, but their life is shortened in warmer tanks, so a chiller is ideal for them. It is very rare to find Bimacs for sell as they aren't legal to collect for sell by California state law. However their babies are, so if someone captures a gravid female and raises her young, the babies are fair game for the market. You may find captive raised or tank bred baby Bimacs occasionally.
www.marinedepotlive.com lists them on their site along with 3 other kind or at least 3 other locations they receive octos from.
The Caribbean species is usually O. briareus, sometimes O. vulgaris, but could also be dwarf O. mercatoris or joubini, OR O. filosus/hummelinki. Filosus are very fun pets (day active, interactive) and can be kept in smaller tanks.
www. saltwaterfish.com is a good source for them. Their octos always come from Haiti, and so it's a pretty good bet that it will be O. filosus.
Pacific or Indo-Pacific is usually A. aculeatus, and they are typically diurnal and interactive. However there are also other octos that occasionally come from these areas like O. luteus which is strictly nocturnal.
www.liveaquaria.com typically supplies A. aculeatus as their Indo-Pacific 'pus, and
www.marinedeptolive.com also receives them. I don't know what Caribbean species LiveAquaria gets, but considering the tank size they recommend for it, likely O. vulgaris or briareus.
Atlantic would most likely be O. vulgaris, although they are quite common just about everywhere tropical/sub-tropical. Hence the name vulgaris which means "common". Don't let that label at the LFS fool you though... "Common Octopus/Common brown Octopus"
Really annoying.
Keep in mind all of these are just based on the most commonly collected species coming in from these areas and not a guarantee. I saw an O. macropus on Ebay, and it is Caribbean, and also grows as large as O. vulgaris, but is strictly nocturnal. Also, while I'm ranting, avoid "Zebra", Mimic, and Wunderpus. They are WAY overpriced because they are very rare, and being very rare, really shouldn't be collected and exploited until more is known about their wild populations, if ever.
Pretty much the best thing you can do if you're serious about keeping an octoups is set up your tank and wait until you find a good source that won't try to BS you about the particular species they provide. It's a guessing game at first, but once you're familiar with the suppliers you get a feel for who has what. A picture of the specimen available is always helpful if they can supply you with one. If you check out TONMO.com you'll find a list of octos in the Octopus Care section of the forums, and the list contains what kind of octopus and who it was purchased from. This can give you an idea of what kind of octopus you would get from which supplier.