Mine doesn't use an Oring seal.. don't think that I've seen one that did.
You "could" probably order one from Bowling Green customs.. now "southern customs".. but I WOULD NOT... I ordered fuel rails from them.. yes.. they sent them.. but 6 months later I started to get bogus credit card charges from them.. maybe they will ship COD.. otherwise.. you will need something custom made. You could get a plenum spacer and have a stock plenum welded to it.