Natural sunlight causes your skin cells to make vitamin d
This is by far the best source. However, unless you live below the 45th parallel, you need another source. Milk is fortified with vit d. Otc supplements
are also good sources.
Qnlabs.Com sells olive oil based vitamin D3 for $12 most inexpensive and tastes good too. The other form the same company has cranberry with vitamin D3 2000 units.
Answer assisted by David Miller, Medical Student