Digital Comics Museum. This is a site that contains thousands of public-domain digital comics from the 1940s and 1950s, from a wide range of publishers. The variety of comics available for download range from war comics to westerns to science fiction to costumed heroes. The best-known character with many comics at DCM is Captain Marvel (Shazam!).
Your Public Library. Don't forget that many public libraries have surprisingly good inventories of graphic novels and trade paperback collections. If they use the Dewey Decimal system, they may be filed in 741, although many libraries have dedicated graphic novels sections.
Hoopla. This is another option for reading things for free, although the details and availability do vary from person to person, or situation to situation. Hoopla is a “borrowing” service for books, audiobooks, TV & movies, music, and comic books that runs through public libraries. So if you have a library card, and your library system is subscribed to Hoopla, you can use the service. The library system that I'm a part of allows the borrowing 15 items in the course of a month, each for a 21-day period. Those details differ greatly from system to system. Because of Marvel Unlimited, there are very few (if any) Marvel books available, but there are a good number of DC and independents books on the system.