Try the following:
The Logoserver. The quality isn't the most consistent, and they're a bit smaller than the 150x150 that OOTP uses, but they're the most complete site out there.
Thee Logo Shack. The logos are a lot bigger, but not always the best quality. Fewer to choose from, though.
Chris Creamer's Sports Logo Page. At the moment, Chris doesn't have anything up in his football section, but you can always keep checking back. The best combination of quality and quantity.
The Sports Logo Encyclopedia. Currently undergoing a convertion to a "members only" site, so nothing's accessible, unless you want to try hack his site. He has vector versions of a bazillion logos, so they look terrific. Hopefully, his site will re-open to the public.
Logotype Free. This Russian site has vector logos, but not a complete set. You have to search a bit to see if they've got what you want. Again, the vector logos are great, since you size 'em as you want 'em.
The Society for Sports Uniform Research. Not a logo site per se, but their "resources" page contains links a whole bunch of logo sites.
All of my MLB logo sets were created with logos taken from the SLE and Chris Creamer's sites.
Hope that gives you some starting points...
--Eriq.