This listing has one:
http://www.vintage-computer.com/vcforum/showthread.php?14000-Various-vintage-parts-for-sale
Looks like he just posted it on Jan. 25th...oh, 2009. It's not 2009 any more is it? Ooops. Well, I'll post this message anyway, because you might want to take the long shot.
Oh, but Herb Johnson's page says the following:
"Video cards for the SE: ask about pric.
Note: unless noted, these SE Radius cards need a Radius Magic Bus adapter 630-5010-A for the SE, which we have available."
So you may wish to check with Herb.