Blue Ice cod liver oil is supposed to be an excellent brand and has a high naturally occuring vitamin A and D levels. It has been tested for mercury and PCBs and is apparently completely clean from it.

But again I cannot vouch for that not having used it becaue I am in another country and cannot get the stuff. But I know many do take it.

Other than that, natural sunlight is a great way of getting vitamin D. You only need to spend about 15 minutes in natural light daily, even just exposing some skin, face, arms etc. But in winter time, it is usually advisable to take a cod liver oil supplement.