The REI Co-op Roadster was an awesome tent! I'm stoked to hear that you're still putting it into service. We're sorry that you've misplaced the pole! With the details that @Gary2 provided, Tent Pole Technologies should be able to replicate the pole for you. As @REI-JennyA suggested they are great at repairing tent poles, but also can build an entire new pole from scratch. I also wanted to call out to your attention the REI Co-op Flash Air Tent, that is a distant cousin to the Roadster. It's kind of cool to see how tent technology has evolved!
Hope this helps, thanks!