are you talking about semaphores?
like these, mounted behind the cab and they swing out when activated
the only place I have ever seen them for sale is in the priceless aisle at the unobtanium store
sorry, thats my poor attempt at humor. I see them for sale very rarely on ebay and they are always priced well out of my realm of affordability. enough that I hope I trip across a farm truck with a set.