As sun exposure in the middle of the day can have harmful effects to fair skin individuals, supplements or dietary intake is often preferred. There is not enough Vit D in natural food other than oily fish. Plain Vit D3 supplements (1000iu/tablet) are a very easy and safe option in controlling adequate blood levels.
Beware of multivitamins and fish oil containing Vit A/minerals. Those supplements can get toxic very easily when larger amounts of VitD supplements are needed.
Please ask your Doctor for advice.