quick question on over flow tank?

I don't see where anybody is making them new. Keep checking ebay and your local Craig's list. We get Darts being parted out from time to time. Check local bone yards even the four door models. Post on the various mopar forum buy and sell threads. Somebody has one out there. One will show up. In the mean time you can buy a cheap aftermarket one to retain your coolant and replace it when you finally find a factory one.