Try the Stant 10825 cap - remove the black plastic outer shell, and the innards will mount in the Vic gas cap.
Did it myself last September for my '99 - easy fix.
However, if you've lost the cap entirely you can get replacements from Victory, Kuryakin, and others; it's been reported that a HD Sportster cap will work, as well, but I've no details on that. The Stant cap may work unmodified, but the appearance may not be to your liking.
'99 V92C with K&N, PMRT chip, Vic stage 1 slipons, and engine by Rylan Vos!
Last edited by Colorado Expat; 05-06-2012 at 12:48 PM.
Reason: expand on answer