The best way to get B12 and D is through supplements. There are chewable ones, which I recommend.
You CAN get vitamin D from the sun, but it's not a good idea to rely on that because:
1. You should stay out of the sun (skin cancer)
It's much healthier to take a pill
and:
2. Certain places and times, there's not much sunlight, like at very high or low latitudes, or during the winter (or in cloudy places).
B-12 is in fortified foods, but you shouldn't rely on that because it's unreliable since it's hard to measure. There's actually not much at all in cheese, egg or butter, or meat for that matter; it's mainly in liver and feces.
Many meat eaters also have low B-12 levels, unless they eat a lot of organ meats or feces. You're probably deficient now, and don't know it.
Other apes, along with most other terrestrial and mostly herbivorous non-ruminant species eat small quantities of their own feces as a supplement.
As a particularly strong example, rabbits produce two kinds of poop: one is eatin' poop and one is leavin' poop; they have very different compositions, moisture content, etc.
We silly humans stop our babies when they try to eat their poop for some reason. OK, maybe that's a good idea. A pill is less likely to challenge your immune system with potentially deadly bacteria.
Vegans in third world countries who don't supplement probably get all of the B-12 they need from contaminated water (you know, from the rivers and streams of murky brown water that they shit and bathe in,
and drink from).
This is also not recommended.
Just take a pill.
