To replace a valve stem on a tire, you need to break the tire's beed from the rim. This requires a lot of pressure and I don't recommend trying it without the proper tools or equipment.
As far as the stem, any tire shop would have them. I'm sure most parts stores will have them too.
Once the tire beed is broken, the valve stem is installed from the inside of the rim and pulled through the front until seated.
Wednesday, March 2nd, 2011 AT 1:14 PM