I'm guessing that the reason you don't see them is they are really not necessary. Drill a 1" hole directly in front of your valve stem, at approximately the 9 o-clock or 3 o-clock position. Buy a 1" chrome snap plug and a valve stem extension. Put a mark on your tire at 6 o-clock when the valve stem is aligned with the hole. Simply snap off the plug and insert the extension, screw it on, and you are all set to either check the pressure and/or add air.