The cup cars use standard glass, same as road cars.
I bought a couple of pieces from renaultsport (rear sides) as they have the hologram stickers on but the part numbers on the glass are the same.
If your looking at those on eBay, they are not any lighter than what you already have but he thinks they are, the 172 cup had lighter glass, perhaps he's thinking these are the same. the reason the renaultsport parts have the hologram stickers is so they can see your using genuine parts, the stickers say 'no paint'
If your wanting lighter, the lexan/poly is the way to go.