A high quality source of vitamin D is liver. Cod liver oil is a great way to supplement your diet. In Iceland you can get it in small sardine type cans and spread the liver on crackers.
It does contain some vitamin D, but it also contains about 10x as much vitamin A. Hypervitaminosis A is a real problem. I would not think it particularly safe to use cod liver oil for vitamin D supplementation because of this risk, especially when other supplements exist.
