Quick search turned these up. Probably depends if you want an all original looking gauge with proper face color and John Deere name on it.
http://www.steinertractor.com/ABC239-water-temperature-gauge?&TF=CDA5082D3E23
http://www.elmersrepair.com/er-ar490r
http://www.antiquetractorsrus.com/johndeeretractorparts/gauges/jdgauges2.html