Are we talking about the same thing? A valve extender, for use with deep dish carbon fiber rims, weighs about 4 grams, and would not be available any Auto parts store I've ever seen. (Excuse me sir, I need the Valve extender that goes with the Zipp 1080's?)

A brass adapter allowing you to pump presta valved tubes with a schrader pump might be available at some auto parts stores conceivably, and might weigh enough that you'd feel it, but I doubt it.