When I couldn't find my 9/64" Allen, I used a 3.5mm one from a bicycle toolkit I have. Worked fine.

The 9/64" is probably much easier to find though. StewMac sells a SAE kit with a 9/64" included :

http://www.stewmac.com/shop/Tools/Wrenches/Ball-End_Hex_Keys.html

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And if you have anywhere near enough guitars to call it a collection, grabbing a set of each SAE & metric is probably a good idea.