Here are some of my favorite logo sites:
For "raster" graphics (.gif, .jpg, .bmp):
www.sportslogos.net
www.logoshak.com
www.logoserver.com
www.allthelogos.com (they have vector logos, but they make you pay for them)
For "vector" graphics (.eps, .svg, .ai):
www.brandsoftheworld.com
www.hqfl.dk (Soccer logos only)
www.lalogotheque.com
www.logotypes.ru
www.sportlogo.net