I want to take vitamin d supplement what is the best form to buy and at what dose? Especially that I have insufficient vitd blood levels?

Vit D. All of the otc vit d is d3 which is what you want dose kind of depends on your wt and present level tons of good info at vitamindcouncil.Org my usual dosing formula is ~ 1000 iu/25# body wt get recheck of level in ~ 3 months almost everybody is vit d deficient.
Possible D3. Most people say that d3 is better than d2 but the evidence suggest that either is fine.