Cantilever cable adjustment device
You can get in-line cable housing adjusters. 

A pair is listed on http://www.sheldonbrown.com/harris/cable-hardware-brake.html, part CAH32. I think those are made by jagwire, and they should be available at virtually any bike shop.

edit: those inline adjusters are for derailleur cables only. Use this http://harriscyclery.net/product/qbp-mickey-cable-barrel-adjuster-1061.htm for brake cables. It goes in the cable stop. Being a QBP product, you should also be able to obtain those at virtually any local bike shop.