i tried to get one from e-bay, but ended buying it directly from leica.it was expensive but it's worth the money. i use it all the time.check with them before buying elsewere.
Can't you just file a standard 135 carrier larger? You can smooth the edges with 400 grit followed by 1000 grit sandpaper and paint the edges flat black. I know this may sound sacreligeous to Leica folk but I'm a cheap SOB.