my lbs carries them as well.
Google valve extenders bike and you'll find lots of options. See eg
http://www.amazon.com/Challenge-Pres.../dp/B0026N5N4M
The Zipp type are the least desireable. The valve core has to be left screwed open, and it will sometimes stick making inflation difficult.
Removable valve core type from Tufo, Vittoria or others are the best, if your tubulars (or tube for clinchers) has removable valve cores.
In between are the type that have a core that attaches to the existing core, like this, which is a good option if you don't have a removable valve core.
Another tip: Teflon tape!