Ghost shrimp and brine shrimp are not nutritious food for octopus or cuttlefish. Ghost shrimp are fresh water animals. You can order shrimp from
aquaculturestore.com
Until you know what size and what kind of octopus you have (as well as it's personality...) you are not going to be able to predict what it might eat. You should have a variety of food on hand for when you get your animal. Personally, I would probably go with smaller crabs and shrimp, that way either a big or small octopus would eat them. You could always order larger later...