The
GCD (Grand Comics Database) is a great place to start (where there are currently 174,087 covers!). You should also check out the
Tips and Techniques forum where you can find several threads on where to find comic book covers, pulp paperback covers, etc.
If you are specifically a fan of horror comic books, you might also want to look for fan or reseller websites specializing in horror comic books. Specialty websites quite often have better quality and/or larger scans, and might also have rare issues that aren't yet in the GCD. Another feature of fan websites is often a favorites list, which are often the most dramatic covers, which usually make them good subjects for recaptioning.