Other than salmon...what are the best/most concentrated sources of omega3? I know walnuts have omega 3 but don't they have a lot of omega 6/PUFAs too?
I think seafood is your best bet.
Here's a chart that gives you a list of high omega-3 seafood choices, in addition to omega-6 content. For the top seafood sources, I'd stick to Alaskan salmon, herring, sardines, trout, shrimp and oysters to eat regularly (lower on the food chain so less toxins, more sustainable methods of capture). Mackerel and tuna tend to be higher in mercury and toxins.
Other sources if you want to vary it up a bit/don't like that much seafood (but the 3-6 balance isn't as nice looking as it is in a lot of seafood):