Several assets can be found here and there (for example in the asset release part of this very forum) and sometimes, I'm lucky enough to find good ones for the viking theme. Regarding the models, what you need to know is that you have different types of assets for different types of things. Usually, you'll find a 3D shape and a skin to apply to the shape. The 3D shape can be modified at ease with several settings (bigger eyes, smaller ears, larger teeth, etc.). My "secret" is that I like to apply a different skin than the one planned for this shape. After that, you can modify eyes color separately, as well as try different types of hair, etc. I think of myself still as a newbie, but the real secret is to try things and have fun doing it
For example, here's the exact same 3D shape but with different skin, hair and eyes color: