Have you tried fish (eat wild-caught)? Mackerel is tasty if you don't like salmon. Those two are both very oily, you can eat one or two servings a week and get enough omega-3. B12 is present in any animal product usually so I recommend eating more meat, cheese or other dairy. You really need the full B spectrum, but B12 in particular is the one to focus on.

I am not an expert, experts may disagree for other reasons, but I do know my math and read the studies for my own benefit.