Car Meeples?

Funny thing, but I just finished a game of Thunder Road with my wife and son. My wife kept stacking her car on top of the other when ramming instead of setting them side-by-side. One thing I can say is that it looks wrong. Strange as it sounds, the comical appearance of it interfered ever so slightly with my enjoyment of the game. If two cars are ramming then there is a visceral satisfaction in seeing the figures butting against each other . . . accompanied, of course, by the proper sound effect. Stacking moots the childish enthusiasm that makes these games enjoyable.

I think you'll have a hard time finding the right sort of pieces. I can think of quite a few car figures in various scales that would be suitable, but it would be an issue keeping them from toppling over once stacked. There are at least a couple manufacturers who make white styrene cars like these (I usually buy mine from unscrupulous recaster-types in China, so I don't know who the actual manufacturers are):
http://www.architectscornerstore.com/categories/Model_Building/Scale_Car...

If it is just a small run for your own use, you could buy models with the domed tops and use a dremel tool to shape the underside of each car so that they rest firmly on each other. A bit too involved for any larger project though.