Ye, plumbers merchants might have something, but I remember seeing something on ebay for this kind of thing.
On a similar note, I have been keeping an eye out when following cars to see if there is a good doner car for a rear flat blade wiper upgrade. But so far I haven't seen one that looks like it would make a good nice, neat job. My old man's 1 series looks like it might do the job, but the angle kicks up too great (rather than being a sweeping curve) and wouldn't follow the line of the window.