The best way to ensure that you get enough vitamin D is to spend time in the sun. Glazed windows and sunscreen block the UVB rays necessary to manufacture vitamin D, and tanning beds generally are not good sources of vitamin D, because they are generally calibrated to favor UVA rays rather than UVB rays.
Remember to take proper precautions when spending time in the sun - too much sun can be as damaging as too little, and skin cancer rates are on the rise. Fortunately, even 15 minutes a day of sunlight on your face and hands is enough to benefit your vitamin D levels if you are light-skinned. Darker skinned women will need longer exposure to the sun (up to about 40 minutes for many African American women) because their higher levels of melanin slow down vitamin D production.