Amazon has countless original Superman and Batman comics going for $1.99 each.
https://www.amazon.com/dp/B07JJ67LCT
https://www.amazon.com/dp/B07JJFKKL5
The Brave and the Bold, including The Justice League of America
https://www.amazon.com/Brave-Bold-19...dp/B07JJ8799C/
The Flash
https://www.amazon.com/The-Flash-195...dp/B07JK5THDC/
Also...
Green Lantern
Wonder Woman
Adventure Comics
World's Finest
Showcase
Action Comics
Detective Comics