It is easy and cheap to make fishing power yourself. When I buy fish at the fish counter and they cut out the fillets for me, I always take the cut off. I put this in a saucepan when I get home, pour on water and put in coarsely chopped onions, whole peppercorns and bay leaves. I often use carrot slices and leeks as well.
I let it soak for approx. 30 minutes, before I strain off the power and let it cook on to half the amount, so it takes up less space in the freezer. I pour the power into suitably large cans, which I pick up again and use in sauces and soups.