Vitamin D3 (cholecalciferol) is the form of Vitamin D that is useful to humans.
There are very few foods which naturally contain Vitamin D3. They are ALL animal foods, and ALL contain fat and cholesterol, as Vitamin D is synthesized directly from cholesterol and is stored only in animal fat.
The best food source is cod liver oil. If you don't like the taste, you can get it in relatively inoffensive capsules these days.
Lard (yep, pig fat) is also relatively high in Vitamin D. This is because most pigs are raised outdoors, and are therefore afforded the chance to naturally synthesize Vitamin D. Unfortunately, if you're anti-saturated fat and religiously trim the fat from your pork chops, you're not likely to get enough Vitamin D this way to make much difference.
The yolks of eggs naturally contain Vitamin D, although most chickens are raised indoors and will not have MUCH Vitamin D. Eggs from "pastured" (as in, ACTUALLY raised outdoors) poultry will be higher in Vitamin D.
Butter and milk contain some Vitamin D3 naturally, but the total amount will depend largely on how much sunlight the cows/goats/other milk-producing animals receive.
Vitamin D3 is synthesized and added to commercially produced cow's milk, as well as to some milk alternatives, and, occasionally, to orange juice. The sun is also a very good source of vitamin D. Our skin makes it as a reaction to the sun, so 10-15 minutes in indirect sunlight a day can also help.