Completing list of refurbished Canon super telephoto lenses available right now are the Canon EF 600mm f/4 L IS and 800mm f/5.6 L IS Lenses
in stock at B&H
also has an unusual number of used Canon lenses
available right now (look for the "Buy Used" links on the right side of the page). I'm not a big fan of buying used lenses (though I love refurbished gear), but a used lens transaction does not get safer than through B&H.