The only MFR I found for that size is Pirelli and they supplied those to Bentley for their Continental S and T models. That said they're hard as hell to find. You may try taking a trip to a Bentley dealership or try this website and see if it has still any in stock.
Alternatively you could try different tire sizes that come close to that in size but remember to stay within 4% variation from OEM. So something like 295/45/18, 305/45/18 or 315/40/18 would be good if they don't rub against suspension bits.
Use this. It's a nifty tire/wheel size calculator that gives you precise measurement and visual diagrams to help with the choice. Everyone looking to plus size should use this tool.
285/40/18 is teh size you should be getting, its much closer to stock size, actually its the size the gt500's run in the rear. i have this size bfg kdw's on my car, and they look great, good amount of sidewall and i usually like the lower profile look( stepped down from 20's to the 18's)
i dont think you will find a 285/45 anywhere with the exception that walter mentioned
sweet...... thanks guys ! The tire link thingy is very very handy, i have added that one and the gear ratio rpm one to my folder of important **** ! LOL