Vitamin D is available across the counter with all medical stores. It is also available in all online stores such as Amazon, Walmart etc thanks to the Vitamin D deficiency which is now Pandemic. Doctors prescription for Vitamin D supplement is also rampant everywhere !
Self medication is also wide-spread due to awareness among people on D deficiency and the blood test for D is available in all labs . This article written by Dr Preethi Raj, PH.D , a Chennai based Nutritionist seems to be a comprehensive article on vitamin D , it's importance, natural food sources, Pitfalls, and correct and sage dosage.
This is only for reference and should not be taken as a prescription. Please consult your physician before medication..