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.
A similarly surprising fact is the Omega-3 fatty acid actually comes from algae the fish eat. As a result there are vegan-friendly algae-derived supplements as an alternative to the popular fish oil ones.
On a brief vegetarian experiment I used a methylcobalamin sublingual B12 supplement, but it had strange and totally unique side effects. After a week of daily use, and this was consistently reproducible, my neck would become very stiff and painful to turn. I don't know why, but it didn't take much of a leap for me to connect this stuff diffusing into my system from under my tongue and the surrounding neck tissue being effected. I find it preferable to just eat sardines regularly.
If you're referring to the Sardines, if you can handle the taste of room-temperature fish broth, drink the juice from the can, don't drain it. That's where most the omega-3 fatty acids end up from the cooking process.
Some of the better brands like what's found at Whole Foods here in CA will even include a notice on the package about not draining the liquid. The Henry & Lisa's stuff [1] in water is quite good, but can be relatively pricy.
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.