Because we use the caterpillar links, we only need to empower one wheel on each side. The rubber helps to keep Vertigo from sliding down. The magnets are very powerful normal to the plane, but they don't apply a lot of tangential force. Because of the high friction coefficient of the rubber, the problem of sliding down is minimized.
The wheels are pretty easy to find in any model building shop. You can find them in all sizes, weights and colors. Make sure to pick some that are strong and rigid, because they have to withstand a fair amount of force. You want to make sure to buy wheels of a big enough diameter. This way, you'll have plenty of space below your base, to attach and govern the magnets. We used wheels of approximately 5cm in diameter.
Wheels in detail:
View on the caterpillar links:
