Mike, In the event you did not find a stall warning switch yours can be overhauled by Safeflight at 914 946-9500 or API (800 950 0111) may still have some in stock.
Are you reasonably sure the problem is actually in the switch? Switches are so expensive that it pays to diagnose first. With power on the aircraft have someone hold the switch up while you gently tap on the horn with the handle of a wooden screwdriver. If it comes alive let it blow for a minute or so. Stall warning horns don't get much activity except at annual (I hope). If you are not successful take a voltage reading at the horn, check for a proper ground and repair as necessary. If you do get the horn working and the sound is not proper it can be "tuned" in place.