Thrift store goldmine.
I spent way, way too much time at Show and Tell Music last night. (I stumbled across it while searching Google for Bettie Page photos.) If you have even the slightest interest in weird record cover art I strongly suggest you check out their galleries because there's some truly amazing stuff here.
A somewhat similar site is Pulp Morgue, which features many examples of old "disposable" artwork like album covers, movie posters, book covers, etc. [Link via J-Walk.]