Aaron Albert points us to ComicSeeker.com which allows you to do some one-stop comic book searching. It actually turned out to be quite useful, in fact, since my father is on the hunt for some old SuperMagician comics. If you don't know SuperMagician, that's because they're actually pretty hard to find even if you're looking for them! In any event, a quick scan using ComicSearch and I came up with a dozen or two issues for sale, and one of them even turned out to be something Dad was looking for!
Definitely worth checking out if you're in the market for Golden and early Silver Age comics.