You kind of need to get your mind in the right frame before you start and come to grips that they have a short life span. I guess it depends on how much you let yourself get attached to them.
The comon octopus in the trade on average live any where from 3-9 months by the time there big enough to be sold. Of course this depends on species. Diet, water temp and quality all factor in too. In the wild they can live up to about 7 years for some arctic species. Most species average between 12-18 months.
Batyhypolypus arcticus, a acrtic octopus is often caught by accident by fishermen dredging for scallops.
The only way to aquire one is to talk to a fisherman from new england and have them keep a eye out for one when they unload their nets. They live in very deep,cold waters and are hard to come by.