I have looked for this myself with little luck; no real "vendors" I've found.
However, awhile back there was a guys elling them on eBay, a home-made kit deal. Unf I can't find it now, though.
It seems that the way it worked was basically taking out the central plastic "knob" peice that's tehre, and replacing it w/ a lock cylinder, so that it usedt he original arm to clasp shut or something.
If you find one, please let me know...