I'm a raw feeder, I've used canned sardines once a week (used Tiki Cat's Sardines), but currently using fish oil into their meals. I don't think feeding canned, cooked sardines that have no added salt and packed in water is bad, actually. I just don't make it a daily thing. I've fed canned sardines as 1 meal (measured 3 oz) once a week. You definitely don't want to feed any raw fish to your cat. Raw fish has parasites.
Btw, I'm on a yahoo raw cat forum, members there DO feed canned sardines (no sodium, packed in water), mackerel, herring, anchovies once a week or they add fish oil supplements into their food. The only other way that I'm aware of is to feed grass fed meats (beef, lamb, venison, etc) or wild game. If you choose wild game, you'd need to freeze the meats for several days to weeks, depending on the source. Oh, you could also add in free-range, pastured eggs (yolks only), those would be great to up the Omega-3s as well, but I've never fed raw eggs myself.
If you choose fish oil/krill oil supplements, be sure it doesn't contain lemony flavors and are soy-free.