Replacement shower wheel?

Hello,

One of the wheels on the top of my shower door is broken. I need a replacement, but can't find exactly the same thing on the web. I don't have any information about make or model (the shower was fitted before we bought the house). There are photos of it here:

http://www.flickr.com/photos/lizandcormac/5943383944/sizes/l/

The dimensions look like about 14mm diameter x about 5mm thickness, or 5/8" x 3/16".

(I just need the wheel, not the whole assembly)

Thanks very much for any help!