Plain Box Rail 8' $33, 10' $54 via Depot - They get you on the cars...they're like $30 or so ea. You may also find them via a supplier of metal pole barns. Many use sliding doors and these tracks.
If they're not available near you, I'd consider using a piece of 5/16" steel flat bar and some generic #4 v-rollers to make my own car that sandwiches the steel. Then weld or bolt it as your configuration requires.
-B