What is the best natural source apart from sunshine to help keep the vitamin d levels up?

Fish: Fatty fishes (tuna, mackerel, catfish, salmon) are one source. Eggs have some; beef liver has a little. Cod liver oil has a lot. After that, you're looking at fortified dairy products or cereals that have vitamin d added to them, or vitamin d supplements or multi-vitamins.

Fortified foods: The flesh of fatty fish (such as salmon, tuna, and mackerel) and fish liver oils are among the best sources. Small amounts of vitamin d are found in beef liver, cheese, and egg yolks. Some mushrooms provide vitamin d2 in variable amounts. Ready-to-eat breakfast cereals often contain added vitamin d, as do some brands of orange juice, yogurt, margarine and other food products.

Meat protein: Vitamin d is really a hormone and as you say, we were designed to live under the sun. It is poorly absorbed by mouth but the best way to keep it up is with a supplement of vitamin d3. Ask your doc re dose.