Sunlight of course is the best source of vitamin D. Unfortunately, few foods have D, and most of them are animal-sourced, like egg yolk, beef liver, and fatty fish. Certain mushrooms, like shiitake, have vit D, and also tofu.
Regarding supplements, there are two kinds of vitamin D: D2 and D3. Either one will do. D2 is plant derived; however, it takes much larger doses of D2 to get enough. D3 is the best way to go; however, most D3 supplements are animal-sourced. Only in recent years have plant-based D3 supplements become available, so, if you supplement with D3, be careful about which one you buy.
I live in a cloudy area, plus I'm not exactly the outdoorsy type. I presently supplement with the D3 shown below. It's available as 1000iu or 5000iu. I use the smaller one. (800iu daily is recommended for adults.) It's about $12 for a two-month supply. There are other brands available too. If you're currently low on D, and you decide to supplement, you might want to double-up on the doses, until your levels come up.