What are some good places to find public domain comic scans?
Lots of good stuff, in a variety of genres, at Golden Age Comics Downloads. It's a forum that allows registered users (registration is free) to download and upload public domain Golden Age comic book issues. The mods are stringent about verifying that all posted comics are in fact public domain. The only drawback is that there are no online previews of any issues; you have to download them to read them.
Examples of interesting titles I've found here are Pep (where Archie started out); Crime Does Not Pay; early issues of Captain Marvel and related titles; Young Romance; and old newspaper strips such as Little Nemo.