resetting or changing out gauge

Dan: you should write a book! The car in question is the 63 valiant. The other gauges do work well and I believe they are accurate. I have had the car for only a year and a half so I don't know all of it's history obviously. The gauge looks to be stock, the engine is original. So where do I get a resistor to put in the temp sender wire? I can live with the gauge reading where it is but it is one of those little things that bug me and more importantly keep my eyes off the road.