I replaced mine from Jim and Bob at LPX years ago, I believe they ran out. Call Chris Parts. Also check out Outwater Plastics... http://outwater.com/lg_display.cfm/page/A-40/catalog/2016_Master_Ca... ....Is this the proper material , I dont know. Let us know what you finally use.
Beckson "Channel Mate" white plastic window tracks are direct replacements for the original window tracks that get brittle and break with age. They come in single or double track versions. I just ordered some from iboats.com for $25 each (white, double, 1/4" glass). They come in 8 foot lengths and some vendors charge extra for oversize shipping. But iboats only charged their standard $7.95 shipping/handling fee.
Here is the link to iboats: https://www.iboats.com/shop/beckson-channel-mate-double-window-trac...
And the link to Beckson: rhttp://www.beckson.com/channel.html
My 31 needs 3 tracks, 2 glass 1 for the screens