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[Ishootforblood]

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[TRAP55]

You might get lucky on ebay, or scrounging through parts boxes at a gun show. Your best bet is to contact Joe at http://www.aperturesnmore.com/contactus.sc and have him make one, if he doesn't already have one.
I've had him make some hard to find thumb screws for quick detach peep sights. The cost is fair, and the work is outstanding.

[kenl]

You can always try the local gun store. I've had very good luck finding weird screw sizes at the one in town.

[Mustangsally5.0]

Use a lighter & heat it up

[kenl]

Use a lighter & heat it up

....or use a hair dryer