You can purchased Jack Mackeral..... wash it first, because it’s salty, but the Jack Mackeral is low in mercury.. another thing that is good is sardines... for our dogs and cats a sardine a day is good.
They do like tuna--and when the human-grade "light tuna" is on sale, it's as cheap as the dark red tuna for cats, so I give them that too. I've been mixing the stuff with leftover diced chicken.
I also get canned salmon from either the 99 Cents Only Stores, from Big Lots, or other cheap places. They eat a little bit of it (so finicky)!
What they really like in canned salmon is the Wolf & Bear brand sold at Costco.
What they go nuts for is when I get hold of some fresh or frozen wild-caught fish (cheaper than farmed when it's on sale), they get portions too. My significant other can't stand salmon but I love it, and it's a real treat for the kitties when I cook the good stuff for them.
I also sometimes get them sardines canned in water, and they eat those every once in a while. Feeding them fish every day is supposedly not good; but they get the stuff usually twice a week.