I'd mostly used the print-on-demand and independent bookstore site lulu for indie rpg stuff in the past, but I've noticed there are a number of vintage comic books and comic strip collections available there. Some are public domain, some are likely of unclear copyright status, and others are perhaps charitably classed as "abandoned" to various degrees.
Here are a couple of places to check for a number of 40s and 50s comics.
The Comic Strip Appreciation Group has a number of restored, vintage strips collected on their page.
