Country Life has a good brand as does Kirkland brand by Costco.
I'm shocked that your doctor didn't give you a prescription for Vit D. My levels were in the double digits, and I am on 50,000 IU every other week. It's a generic, so you could probably get it for $4.
I doubt that it was the cause of your prior issues, but D has an effect on the immune system.
Please go to DR Khalsa Soram and buy his Vitamin D3. Now days vitamins are not what they say. Dr Soram has a lot of information on Vitamin D3. I have a thyroid issue and low on Vitamin D3. This doctor is integrated natural doctor. I only buy his! he was on DR OZ and well known. loony56