Does anyone know where I can find replacement for this mini-blinds part?

I am looking to replace the plastic valance carrier frames for my Levolor aluminum mini blinds. The old plastic frames have yellowed with age. Each frame holds one mini-blind slat, and clips on at the top of the blinds to form the valance. I cannot find this replacement anywhere on the Internet, including the Levolor website! The model of blinds is Levolor "Monoco" Custom, dating from the mid-1980's. Can anyone direct me to a source? Many thanks!