http://www.hard-to-find-parts.de/" onclick="window.open(this.href);return false;
They also sell on eBay.
I was quite impressed ! High quality, well made and while not cheap, if you don't want to replace a leaking one with something else or if you can't buy one new then I think these are not a bad option to know about. Volker Schultz is the owner, and I found him to be very good to deal with. Below is a photo of both the kit he sells (which has the flat sealing washer, new spring, new screws and some other bits) and also the new diaphragm for my T500.
