Aha! You have *clamp-on* downtube shifters. So what you need is a clamp-on downtube cable stop. They are also sold by Harris:
http://sheldonbrown.com/harris/cables.html ... scroll down until you see this picture:
These are fairly ugly and $12 is quite expensive. I have several of these lying around in my basement, and they are of a much nicer low-profile style. If you tell me what the diameter of your downtube is, I can find one that fits and mail it to you.