I've seen giant octis in public aquaria with total replacement water filtration/circulation. In a home aquarium, the species would require massive and frequent water changes due to the messy nature of their feeding habits. The tank would need a sizeable cooling system as well as a locking cover since even a small 1 pound octopus can lift a 40 pound lid.
To get one, you'd have to either catch it yourself, hire someone to catch it, or test your luck in checking returning fishing boat catches between Seattle Harbor, along the Strait of Juan de Fuca, and through Coho to the Pacific. The last option may prove easier if you posted a reward for a live specimen in local circulars/bait shops/etc.