The best form of D3 that I've found is Bio-tech Pharmacal's which you can get at about half-price from Amazon.
It is a dry capsule that comes in 5k or 50k doses and should be taken with a fatty meal.
Often it is the oil in the d capsules that makes a person ill. Often the oils used in processing are subject to heat in storage or processing making them rancid...if they are not already rancid to begin with. That's why I like the dry capsules taken with a fatty meal.
Vitamin D is added to milk but so is vitamin A. They are added in a oil solution before homogenization.
Also it is EXTREMELY important to take magnesium when you are taking vitamin D3. You can take vitamin D3 in a larger dose weekly but you MUST take magnesium daily.
The best and most absorbable forms of magnesium are chelated magnesium and magnesium oil.
You also MUST make sure your vitamin D IS D3 and that the back of your label doesn't just list vitamin D or specify that the vitamin D is from vegetarian sources.
Vitamin D3 can ONLY be derived from animal/fish sources. If the vitamin D is vegetarian sourced it is D2 and is a much more toxic and less effective form of vitamin D.
Additionally, any vitamin A you take MUST NOT be beta carotene NOR preformed vitamin A/vitamin A palmitate.
Beta carotene is a vitamin A precursor and is only converted at a 12 to 1 ratio by the body...some people cannot convert beta carotene to vitamin A at all.
Preformed vitamin A/vitamin A palmitate is manmade vitamin A and is 10 times more toxic than natural vitamin A. Natural vitamin A is made from fish liver oil. Look for "(from fish liver oil)" on the back label.
Do NOT give beta carotene to children. Children are not able to convert beta carotene to vitamin A AT ALL. Children must have natural vitamin A for growth.
Unadulterated cod liver oil is a good source of both vitamin D3 and vitamin A. However, READ THE BACK LABEL. Many companies are now putting manmade vitamin A in their cod liver oils to maintain D/A ratios (what they say) or to cut costs (what I say).