The primary source of vitamin D3 is sunlight exposure. When your skin is exposed to sunlight, it synthesizes vitamin D3 in response to the ultraviolet B (UVB) rays from the sun. Vitamin D3 can also be obtained from certain foods, such as fatty fish (e.g., salmon, mackerel, and sardines), egg yolks, fortified dairy products, and some mushrooms. Additionally, vitamin D3 supplements are available in various forms, such as capsules, tablets, and drops.