You can't buy it as a seperate part. The trim/moulding is apart of the glass. Hell, for the removal if the glass, it requires a guitar string to remove it.
You have two options.
1) buy new rear window(s) - about 250 for each
2) Bag the area, tape it down, sand, spray, clearcoat
I am planning to do option 2 this spring or summer.
Familiarize yourself with the link below.
http://www.mx6.com/forums/2g-faq-app...tted-trim.html (How to - improve quarter glass trim (repair pitted trim))
If you live in a cooler climate, like me, the results will last longer than hotter climates
I also found that standing water on the trim contributes to the deterioration.