Velomine has them for $15...Ben's good people.

1) Run a tap thru the female end to chase the threads.
2) Make sure the washer is seating in the recess (if the Merckx frame has one) or replace the washer for one with a smaller OD.
3) Grease the male threads.
4) Make sure the sex bolts are not bottoming out, if they are...get a different bolt, or file the female half down in length.