I usually just buy a selection of marine fish from the grocery store, and process in my handy cuisenart with a dash of golden pearls and some nori, press flat in ziplock bags and freeze. I add Selcon if I have any around.
A few things I have learned over time-
Fish LOVE squid. It is easier to chop in the food processor if you get it half frozen in the freezer, and cut it into chunks first. And it is one of the cheaper seafoods.
Raw shrimp are great too, Usually remove most of the shell first because the fish never eat the shell and it makes for alot of waste in the food.
Also, here in Iowa, Alot of the fish they sell is not so fresh, but for some reason the salmon is usually very good. You probably will have better fish where you live.