There are many different sources out there, but the best ones are fermented. The fermenting process exracts the nutrients without the use of heat- so you get more of the nutrients. Plus, it makes it easier for our bodies to digest and absorb.
Personally, I use Green Pastures Fermented Cod Liver Oil and Butter Oil. The Butter Oil adds a whole new host of nutrients for your body.
The price looks mighty steep, but you only take 1/4 tsp a day! The jar lasts forever! And they come in these flavors- cinnamon is my favorite.
You can click here for just the Cod Liver Oil, or click here if you want to look at the Cod Liver Oil/ Butter Oil blend.