The two major forms [of vitamin D] are vitamin D2 and vitamin D3. Vitamin D2 (ergocalciferol) is largely human-made and added to foods, whereas vitamin D3 (cholecalciferol) is synthesized in the skin of humans from 7-dehydrocholesterol and is also consumed in the diet via the intake of animal-based foods. [...] The differences do not affect metabolism (i.e., activation), and both forms function as prohormones. When activated, the D2 and D3 forms have been reported to exhibit identical responses in the body DRI - Institute of Medicine of the National Academies
with that being said, in the same text we can find that
There are a few naturally occurring food sources of vitamin D. These include fatty fish [such as salmon, tuna, and mackerel], fish liver oil, and egg yolk. Some foods are, however, fortified with vitamin D.
for example in the United States, fluid milk is voluntarily fortified with 400 IU per quart
The Food and Nutition Board has established the Recommended Dietary Allowances (RDA) to make nutrient recommendations Dietary Reference Intakes (DRIs)