cyclist/gearhead/cycli...
Join Date: Oct 2004
Location: DC / Maryland suburbs
Posts: 4,166
Bikes: Homebuilt tourer/commuter, modified-beyond-recognition 1990 Trek 1100, reasonably stock 2002-ish Gary Fisher Hoo Koo E Koo
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You'll have to remove the bearing balls and measure them to be sure!
MOST headsets use 3/16" balls. Hopefully the bearing races themselves are not damaged, since these will be much harder to replace.
Get grade 25 balls (the best, lower number = better). Replacing headset bearings is a very cheap and very worthwhile maintenance. You can certainly buy these from your LBS rather than mail-ordering them. Every bike shop and its mom has them, and they're usually cheap (I think 5 cents/ball at my LBS).