Clownfish, you've never told us where you live. Are you in the US? Are you along the coast? We might give you different advice depending on where you live.
Ghost shrimp are freshwater and live only a short while in saltwater, so buy them when your octo will eat them right away. Small hermit crabs and snails might be the way to go, supplementing with pieces of thawed frozen shrimp, assuming your octo will accept frozen food.
You should always have food on hand, not plan to buy it every few weeks. You could keep fiddler crabs in a plastic container or a turtule bowl, use half salt water and half RO water, given them some rocks to climb out on. They will eat good qualilty salt water invertebrate food (flakes).