Did you google "share the road car magnet"?
http://www.supportstore.com/car-magn...-the-road.html
Your options go up if you don't mind a sticker. Chances are these will be easy peel and come right off, if not, hair dryer will do the job.
http://www.cafepress.com/+share-the-...umper-stickers
http://www.sharethedamnroad.com/
If none of those work, just start your own cafe press store and make your own.