http://www.ifirobotics.com/robot-traction-wheels.shtml
Thats the website, you can buy just the tread
Its known as incline conveyor belting and is available from several places, most commonly afaik mcmaster car
just search "incline conveyor belting"
http://www.mcmaster.com/#incline-conveyor-belts/=19to16
both wedgetop and roughtop are available, roughtop in different types of rubber