You've probably seen this corrugated stuff, it's very popular nowadays for ads. The best way to describe it would be corrugated plasticboard. It literally looks like plastic cardboard, but it's far tougher (in both durability and the ability to cut it easily). Gas stations are a good place to find it. The product is also the height of standard HO roadbed. I've had great success in using it for 3d backdrops, townsites and stretches of roadway both straight and curved. You can sketch on it with pencil or pen, and it takes paint of all types AFAIK (I used latex acrylic, it would need 2 coats of it, haven't tried to use oil based yet).
In finding this commodity I learned something else. The thin signs often seen at gas stations are made of STYRENE! Yes, like Evergreen styrene. Like the stuff we use to build models. Gas stations throw it away when an ad is expired, so if you have a friend who happens to manage a convenience store/gas station, he or she could probably get you as much as you need. The only drawback to it is that it is often photo-printed on. I suspect a paint job with a styrene compatible paint would take care of that rather handily.