thanxrfactor central (need to register to see most download links)
http://rfactorcentral.com/
VirtualR often have download links when they cover a mod.
http://www.virtualr.net/category/rfactor
Check this very forum. The Racing Club threads have links to the car mods and tracks being used for a particular race and you know they'l be good quality.
http://www.racedepartment.com/forum/forums/rfactor-racing-club.14/
I'd also recommend EnduRacers Endurance Series mod, one of the very best.
http://www.enduracers.com/downloads_es.html