This is an amazing collection of sports logos.
It has a detailed history and examples of primary logos, wordmarks, and uniforms for literally thousands of sports teams around the world. It even has a bunch of speciality and event logos, like all star games, or memorial patches.
I'm not sure if it would be a whole lot of help for poster stuff, but im into this kind of thing and use that site all the time. It has some great insperation for wordmarks, and would be great for someone designing t-shirts who gets tired of using just brush script and a tail on "uniforms"