http://www.redrock4x4.com/shop/categories/228_244.html
http://www.midwestcontrol.com/catdisplay_short.php?pg=228
Here's a couple of links to show you what they are and what they look like. Probably if you look in your local yellow pages you can find a listing for a store that sells racing equipment. Lots of those performance shops that sell dirt track or auto racing stuff sell them. You may even be able to find them at a good hardware store?