If it's an older RT then the simple answer is no, you can't get a vert body for it.
I've seen pics of a couple of original RT vert bodies and I think all of them are custom made (chopping off the powerfeed and silver-soldering a vert feed on) like skeet said.
Here's a thread with a pic of the one Manike had:
http://www.automags.org/forums/showthread.php?s=&threadid=5437&highlight=manike+rt+vert+body
If you've not bought it yet, I'd definitely advise against buying an original RT if you want to have an upgradable marker.
The slimmer valves with air via the banjo bolt means they're incompatible with every AGD upgrade bar L10. They're good markers in their own right, but only really worth having if you want to keep them in stock condition.
You'd be far better off getting a Retro valve for a classic mag or buying a RT-Pro second hand.