I often search grocery store aisles for decent deals on both canned (in water, no added salt) and fresh fish. Not every fish should be fed (or consumed by you!) but many are safe.
I've heard way too many questionable things about farmed fish, or wild caught salmon that I tend to steer clear of many larger, 'popular' fish. However, my dogs do get a Wild Caught Salmon Oil capsule a few times a week.
Our dogs get a mixture of Jack Mackerel, Sardines, Smelt (not as often), Anchovies & occasionally Trout (when in season). Most canned products have loads of extra salt, but I have been able to find a few companies that only can in water with no salt (look at the ingredients!). Jack Mackerel is harder to find without salt, so I feed that very seldom.
It isn't ideal, but at least its a part of a larger variety of raw meat protein sources for them in their raw diet. My goal is to find and feed whole raw fish at least once a week.
In the fresh variety, I have been able to find whole anchovies, some whole smelt. Mostly, the smelt and trout are 'dressed', aka 'cleaned & gutted'.
Some other items I've been able to find (whole) and feed: prawns, squid, fish heads