a few quick options..
1. buy yourself a can of coke and crucsh that WHEN you finsih drinking. keep it some where handy.
2. Go to a plumbing store and find a PVC end cap.. keep is somewhere handy..
3. Make yourself a small block of wood, keep it somewhere handy.
4. Check your local dealer, they will sell one.. if you lose it start at the top of the list. I think chris at California cycle Works has some of those 'buying' ones.
Personally I try and do the coke can thing.. tie some string to all of them as well.. makes it easy to pick it up once you have already put all your gear on and are sitting on the bike. wrap that string around your clutch lever.
In accordance with the prophecy...
Mal