Try GP motorcycles in Pontypridd - they're breaking a few ST's at the mo'.
If you go the eBay route, don't worry if you can only find a lhd lamp - the only difference between the lhd & rhd ones is a small metal blind screwed in to the polyepsilloidal lens ( the bullseye one for dip ).
Take the dip bulb out & look into the lamp from the back & you'll see it running across the front of the lens - just unscrew & replace with the one from your rhd lamp!
Andy