I did a quick google search and found a couple of possibilities including these http://www.ebay.com/itm/New-10mm-7-8-Bicycle-Handle-Bar-Black-Mirror-Mount-Auminum-Clamp-2-pcs-Hot-/310730602356
They are a complete clamp for a mirror but it is possible that they may be the correct bolt spacing to work as you upper clamp. Worth looking at, but as a always play safe and if in doubt, don't do it.
There are other risers and clamps and again the top half may work for you. Thse are probably too far apart http://www.ebay.com/itm/Universial-Motorcycle-Bar-Handlebar-Mounts-Riser-Clamp-7-8-Standard-/400557105397