Calciferol from doctors is ERGOCACIFEROL Vitamin D2. Human skin, when exposed to UVB, produces CHOLECALCIFEROL vitamin D3. Generally, because there is no conversion required Vitamin D3 Cholecalciferol is the most efficient form.
The case against Vitamin D2 is set out in these two papers.
"Vitamin D2 Is Much Less Effective than Vitamin D3 in Humans"
"The case against ergocalciferol (vitamin D2) as a vitamin supplement"
It is possible to buy BIOTECH 50,000iu cholecalciferol by post from online US sources if you search.
Otherwise and perhaps better, you could buy 5000iu Cholecalciferol oilbased gel capsules which are very cheap and use 10 x 5000iu each week. Alternating 1 daily with 2 daily averages 3 every 2 days or 7,500iu daily. Up to 10,000iu daily (about the amount full body sun exposure skin creates in 30 minutes) is known to be risk free.