The chain on my road bike got caught in the rear cassette and as it snapped it broke one of the guide tabs on the inner cage plate of my new Shimano Ultegra 6800-SS rear derailleur. My two questions are:
The relevant document is Shimano EV-RD-6800-3608A (Jun.-2013-3608A), which gives an exploded view and part numbers for each of the parts of the derailleur. For the Shimano Ultegra RD-6800 SS, you're looking for Shimano Y5XH08000 for the inner plate. (This also applies to many of Shimano's other parts to find replacement pieces)
Given that you've invested in Ultegra level gear, I'd not risk it and just wait for the replacement to arrive.
Bike shops will sell you a replacement. If you are in the UK then for example