You can buy good quality Vitamin D supplements at grocery stores and drug stores. Oil-based gelcaps absorb better than tablets. Buy Vitamin D-3 rather than other forms.
I'm currently using Nature-Made Vitamin D-3, 5000 IU (shown below) during the winter, and it's working well for me. The minimum suggested daily amount for women of my age is 400 IU, but more and more experts are saying that we will do better with much higher levels of Vitamin D.
You may want to ask your health care provider to be tested for Vitamin D levels to determine how much you need to take. If your levels are very low, you may be prescribed large doses of Vitamin D for awhile, such as 50,000 IU once a week for 6 - 8 weeks, then taper off to moderate levels (1000 IU -- 5000 IU per day) for maintenance.
It's not a good idea to take the higher levels (50,000 IU) on your own, since toxicity at those levels can occur over time. In general, though, people are at a far greater risk to be Vitamin D deficient than to suffer from Vitamin D toxicity.