Eat more mushrooms. I do a mushroom-heavy soup about once a week, usually lentils with an entire package of crimini mushrooms sliced up.

Keeping a variety of unsalted tree nuts and seeds nearby to conveniently snack on is good as well.

Edit:

For B12 in particular, it appears Shiitake and Lion's Mane mushrooms are best. They're not competitive with animal B12 sources, but every bit helps and mushrooms don't add much in the way of calories to a diet.

Personally I consume canned wild, unsalted sardines in water fairly regularly for Omega-3s and B12 specifically. They're small and have a short lifespan, which is desirable for a variety of reasons with seafood. i.e. Low on the food chain, and less time spent in a potentially toxic environment accumulating nasties like mercury. Sardines are considered a sustainable superfood by many. It's way better nutritionally than tuna, you eat the whole fish, even the bones.