B&H Photo - Check daily as B&H occasionally has 10-rated (or email-for-condition) reconditioned/refurbished Canon lenses.
Adorama - Refurbished Canon Lenses - Many lenses listed as "Refurbished By Canon USA".
Adorama - The online used Canon lens inventory is generally strong here.
Canon Store Refurbished EOS DSLR cameras - The Canon Store also typically has a nice selection of like-new refurbished EOS DSLR cameras.
eBay - Used Canon EF lenses bring a premium at eBay, but the selection is usually large.
Canon - Canon (direct) often has a strong selection of refurbished Canon Cameras and lenses.
Cameta Camera - Located in Amityville, NY and typically has used Canon Lenses.
FredMiranda.com - One of the most active camera equipment buy/sell forums on the net and rich in used Canon EF lenses. Reasonable deals on used Canon lenses sell extremely fast here - sometimes in minutes.
SportsShooters.com - Many used Canon EF lenses show up in the classifieds section.
NaturePhotographers.net - Login required. A buy/sell forum that usually has many used Canon EF lenses.
Midwest Photo Exchange - Used Canon lenses are often mixed with the new lens listings at this retailer.
KEH - Again, used Canon lenses are often available at this retailer.