water pump block off

I have seen rubber caps for this and used one years back but can't remember who made it.
What I used might not have come from an automotive application.
Some info on it here
http://board.moparts.org/ubbthreads/showflat.php?Cat=0&Number=6488872&Main=6488300

And I might check Mcmaster - Carr or another large industrial supply house for rubber caps or plugs to see what you can find.