eBay
I was inspired by Ken Rockwell's review of this lens:
This is the lens for professional hand-held general-purpose nature and wildlife shooting. If you're taking an African safari, this is your lens. It's also wonderful for portraits, with a long zoom range that will blow-out backgrounds better at 400mm at f/5.6 than a 200mm will at f/2.8 (focal length is far more important to selective focus than aperture).
and I have not been disappointed. The only problem is that the combination of a 1D Mk IV and this lens is just too heavy for my wrists to handle now.
In superb condition, it comes complete with caps, hood, case and box.

I will post fully insured for next day delivery