<<WTF do they make the lense out of? Maybe we could just buy some. Plexiglass is too soft though.>>
Polycarbonate is the material. See here for more than you ever want to know about it.
http://www.psrc.usm.edu/macrog/pc.htm
You can buy it in sheets. Have fun trying to shape it if you can't get stamp it into shape while molten.
the reason they yellow is dirt and heat. The headlamps make the material hot and slightly softer and the road rash (sand, stones, dust) actually embed themselves into it. Then it cools down when you turn off the lights and it hardens with the debris in it.
When you go to a meet, if they haven't cleaned the yellowing off, you can easily spot who leaves their headlights (and fog lights) on all the time and who doesn't.
A simple solution is to continually wax your headlights (after throughly cleaning). that will prevent a lot of the dirt from getting embedded in the lens in the first place.
Doug