The 1973 Dodge models (Polara & Monaco) both used plastic rear valences, probably because it was a one year only rear bumper on those cars and manufacturing was probably looking for ways to cut tooling costs. As a result, if someone driving one of those cars even bumped something with them, they tended to shatter due to being plastic and usually brittle after a few years.
I had a really hard time finding one myself for my 1973 Monaco and ended up having my body/paint guy make one up for me out of metal. But hopefully you will fine one, but it likely won't be easy.