If you have some old ones, you an rebuild them using 2 layers of 6mm foam glued together, and some headlining material. I sewed the new covers leaving the hinge seam open, then hand stitched them closed. Looks pretty good IMHO.
The visors have an internal plastic clamp at each end on the mounting pieces (iirc) to provide resistance when you pull them down. The hinge end of these plastic bits tends to split.

