GCD (comics.org) is probably the best of the online resources, if only because they are continually checking their records and updating the credits on their site. Some other online sites and resources do not update regularly (or at all), so they contain older credits that have since been discovered to be incorrect.
But I can't imagine how any site would seriously credit Severin for that Thor cover, though...just looking at it, it screams John Buscema.
ETA:
Subby #13 is great choice! Always been a big Namor and Dorma fan.