ebay?
Have you checked coolermaster themselves?
If you cant find anything drop them a email, stating you want to spend more money on their products to upgrade or words to that effect. I'm sure they will reply.
If all else fails you could make your own window. Do a google on something like case modding side window and watch a couple of youtube videos. The advantage of this is that you can make the window any shape/design you want and also you can get the plexiglass in a variety of colours. Just make sure you don't cut out too much of the panel otherwise you might weaken the panel.
The problem with doing this, is that me knowing me, i'd totally mess it up, making a total bodge of it...
Thanks for the link though