oh, i know a food source, i dont know how big these guys are when born, but if you go to the beach, that floating sargassum weed is packed with small sargassum shrimp, crabs, and sometimes fish (the fish are a little big for your octos though)
just grab a large chunk and put it into a bucket (i would collect as much sargassum was possible) then when you get home but a heater and a Powerhead in the bucket and whenever you want some shrimp just grab a patch of sargassum and bring it to an empty bucket.. then shake it out really well (replace the sargassum into the holding bucket just in case there is still some shrimp/crabs on it)