Where to get the short wire of the cantilever brake?
I know an answer has been selected, but I wanted to post this here for future reference.

First - the common name for that type part is a "straddle cable" or a "link wire" depending on the variations or the manufacturer.

Second - my primary source (after my LBS) for less-common parts, especially parts for older bikes or the bike-shaped objects the neighborhood kids want me to fix is Loose Screws.

Here is their Straddle Cable page.