You're looking for the Canon Tripod Mount Ring B(w). Since the lenses that fits (70-200mm 2.8L / 2.8L IS / 2.8L IS II, 100-400mm, and IIRC 28-300mm) all come with one, they aren't sold separately by retailers. Your best bet is probably to call Canon service/parts, or post a WTB on fredmiranda.com.
Dont forget about the ebay option... You dont have to get a cheap knock off, see if they have the OEM goods... That's where i got my collar for my 70-200... Dont think i spent more than $30