If it's Knoll...
If it's Knoll, call them: 1-800-343-5665.
My experience has been that if installation requires turning a screwdriver, they won't just mail the part; instead, they send someone out (at no charge) to do it for you... Which can be annoying, but whatever.
If it's not Knoll but you know the vendor or distributor, call them.
If you have no idea who made it, point your web browser at http://www.fastenal.com . Click on "fasteners", then "sockets", and see whether they have something that looks like what you need.
Good luck.
http://www.fastenal.com
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