Anyone who has been following the Weston Price Foundation and has used cod liver oil in the past is probably vaguely familiar with the questionable nutrient properties of these oils, as well as supplements in general. You can read about the rancidity of the oil here: http://drkaayladaniel.com/a-big-dose-of-codswallop/
Dr. Ron heart problems here: https://www.drrons.com/product-blog/Too-Much-of-a-NOT-so-Good-Thing
It really is unfortunate that we are mislead in both nutrient content of foods as well as negative aspects of food from oxidizing to pollution.
The main nutrients people are striving to get in cod liver oil are Vitamin A, Vitamin D3, Vitamin K2, and Omega 3. It really is a nutritionally complete food and makes it so you don’t have to worry about purchasing organ meats or wild caught fish to meet these nutrient requirements.
Alternatives that I personally use are lambs liver and veal brain, if that doesn’t float your boat (and it shouldn’t), then wild caught fatty fish like salmon, herring, or mackerel is a safe bet. (Fish roe/caviar being ideal, but not practical for most) Canned cod liver is of course the best canned option, followed by canned fatty fish.
If you are not averse to dairy or eggs, and have access to high quality, raw, pastured products, you can obtain complete nutrition with much more approachable foods, like scrambled eggs or a tall glass of raw milk.
At the end of the day you can take a few pills if you cannot afford or do not have access to the former. I suggest a cod liver oil supplement for Vitamin A and Omega 3s, a D3 supplement, as well as K2 if you don’t have quality animal fats in your diet.
All of the supplements that I recommend/use are on my amazon shop: https://www.amazon.com/shop/franktufano
The Rosita brand products are available on perfect supplements:
Liquid Cod Liver Oil: http://www.perfectsupplements.com/ProductDetails.asp?ProductCode=RO-EVCLO-5&Click=5becdcfa8a5cd&a_bid=818e12c0
Cod Liver Soft Gels: http://www.perfectsupplements.com/ProductDetails.asp?ProductCode=RO-EVCLO-SOFTGELS&Click=5becdcfa8a5cd&a_bid=2cd6ef64