Mushrooms...not just your mother's hallucogen anymore
My sister-in-law is a nutritionist - one of the ones you usually pay big bucks for, and I get her for free! She will only recommend supplements for people who absolutely cannot get the nutrient from food. The theory is that foods rich in a certain nutrient also contain other nutrients that boost the power of all. That's why people often take megadoses of supplements, never achieving the desired effect on their health. It's all very complex biochemistry, and nutrition has no clue to most of it at this time (she fully admits).
I have just discovered that mushrooms are absolutely loaded with vitamin D - 2700IU in three ounces. Furthermore, unlike other foods rich in D, they are virtually calorie-free. They're acceptable to vegans and veggies and all but the most confirmed carnivore (and carnivores are so politically incorrect at the moment that no one cares about them!).
Anyway, if you just can't seem to get enough D, maybe it would be worthwhile to munch a few mushrooms - cook them, marinate them (oil will add calories), just eat them raw in salads, etc.