For new - find a dealer selling race products clip-ons, inexpensive and do the job well or as mentioned above - stick 50mm in an Ebay search, used suzuki clip-ons came up in my search and from memory I think R1 forks were also 50mm as one point.
Ahhh - here we go; some nice stuff from the USA with an ickle fitting chart
27-Dec-2006, 13:13 
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