You can get a replacement from any parts store. You are looking for a Vented or Closed system. This is a quick explanation of the differences:

https://jimdinner.wordpress.com/2012/07/09/radiator-caps-explained/

I recently turned my open system to closed by adding a recovery tank, so I had to get a new cap from Oreilly's. This would work, unless you are looking for stock look. Similar as the Mopar cap

https://www.amazon.com/Stant-10231-Radiator-Cap-Vented/dp/B000B8N3GE/ref=au_as_r?_encoding=UTF8&Make=Plymouth%7C43&Model=Barracuda%7C2302&Year=1970%7C1970&ie=UTF8&n=15684181&s=automotive&vehicleId=3&vehicleType=automotive