I don't like the idea of just using the valheim world website to find things like crypts, considering the fact that if you want to just see one feature, like how many crypts are in an area X, it shows you the entire map.

But anyways, it is pure RNG, but the best chances to find one is in deep swamps. If they're long but narrow, then it's pretty unlikely to find any, but your chances are better in thicker type of swamps. Of course, you first got to find and explore them, so, not the most useful advice.