You can take over-the-counter supplements

Most say up to 2000 iu daily if you haven't been tested, and 4000 units daily if you are known to be low. It is a fat soluble vitamin, so you should either get the softgels (so they come with their own "fat" essentially), or be sure to take them with a meal that has some fat in it. Carlson is a good brand to get.

If your doctor is very concerned, he/she may prescribe higher doses for you. Some people take 50,000 units on a weekly basis until their levels "catch up".

Interestingly enough, vitamin D impacts ovarian function. Low vit D levels have been linked in one study to a possible increase in ovarian cancer risk (which is a big deal to me since my mother passed away from it).