My understanding was that the ratio shouldn't be more than about 4:1 Omega 6 to Omega 3 - so the average US diet does include way too much omega 6 (though it's worth remembering that that's because the average American eats a lot of processed and fried junk - so if people are already concerned about their diet they may be eating less omega 3).
In terms of other omega 3 sources - walnuts are good, but they have slightly more omega 3 than omega 6 fats. Aside from that, you're basically looking at fish and fish oil supplements. Cooking fish can destroy some of its omega 3 content but (as long as you're not cooking it to v high temperature - e.g. deep frying) oily fish are still a good source of omega 3. There's also uncooked fish - sushi, pickled fish, cured fish, etc. Or you could take more fish oil.
Folks probably already know this - but if you're planning on taking lots of omega 3 fats it may be worth discussing with/mentioning to your Dr. These thin the blood - normally a good thing, but can cause problems with bleeding and bruising, and may increase the risks of certain types of strokes and heart attacks and interact with some medications.
Are there any good (blinded RCT?) studies showing the benefits of consuming large amounts of omega 3 fats? The only studies I've seen on large doses have been re. mental health problems. I know that eating some omega 3 is good for you, but bodybuilders often take shedloads of the stuff...