get the part numbers from your gauges and setup a eBay search. every once in a while NOS gauges show up and not that unreasonably priced.
also set up a search for the cluster as a whole. I have spent 30 to 100 on crapshoot clusters in search of good gauges for my dart. I typically look at the resting needle position, the color of the needle, the corrosion on the rear posts and the cir board. I have been about 50% successful. Of the 4 clusters I have bought I have 4 usable gauges (Dart standard cluster the fuel and temp are the same gauge just need to swap the faceplate. might be the same for ralley but I think not) I also got one NOS temp gauge for a reasonable price. Some here and some on Ebay
Is this right?
NOS Temperature Gauge 1967-71 A Body Dodge Plymouth Rallye Dash Cuda Duster | eBay