If you cover looks like the picture below then this is how you can order another one or two:
They are a Trico Part, 4 inches long, 1 1/4 wide and snap over the mounting bolt on the wiper arms.
Rome Truck parts
800 284 4345
Talk with Kristen or Bill in Parts
Part number AW-RPHC
* If this is not your cap then ask Rome if they can source it for you.
* And this would be a good time to oil your articulating wiper arms you don't see. Also you should tighten up any loose bolts.
Good luck from Polson!