In addition to what others said, I'll make the case for a smaller octopus rather than a larger one (such as a GPO).
- you have a large tank, this is perfect for a small octopus because it will give them room to move around and live comfortably. Most octopuses will find or build themselves a den, and then den up and spend the vast majority of the time in said den. A lot of octopus owners are disappointed that their animal spends so much time camouflaged hidden under a bunch of rocks. This will happen regardless of animal size, species, and temperament. They are solitary creatures with incredibly delicate bodies packed full of protein and nutrients for other animals to eat, and their only defense is to hide extremely well. If your animal is more comfortable in the tank, they will likely spend less time hiding away, and more time exploring the environment or moving from place to place.
-If you put a large animal in a small tank, they won't be as comfortable, so they will spend more time in a "comfortable" position (usually tucked into the corner as far as they can go, with their arms all wrapped around their mantle. Time and time again I've seen aquarium exhibits with a GPO and the octopus is tucked as far away in the back corner of the tank as they can be. This will happen to you with any octopus you get, but the behavior will be more pronounced depending on the ratio of tank size to octopus size.
The bottom line is, larger octopuses require more food, create more waste, cost more, and require more space than you would likely be able to give them, even with a 1000 gallon tank. If your goal is to increase longevity of the animal, and have a shot at seeing them move around and interact with their tank environment, you are probably best off getting a mid-range octopus (O. bimaculoides or Cyanea).
I don't mean to discourage you from owning an octopus, they're excellent creatures and very fun to take care of. I just want to make sure you know what to expect so that you and the animal don't have a negative experience.
Keep us updated if you do find an octopus source! I would love to see photos of your tank setup.