Tried eBay many times, might be worth contacting Silverstone. I thought that at least one place would sell them in uk.
You cannot purchase the side panels for the TJ07 individually as they are all specifically fit to that case. In other words you cannot take the side panels from one and fit them to another as they just wont match up. It has something to do with the process of making the U-Bend in that each one is slightly different.
I bought my side panels from ebay, you could always considering making the side panels your self using acrlyic
http://www.scan.co.uk/Product.aspx?WebProductId=666618 ?? not in stock tho but might be worth giving them an email
Got my replay from Silverstone's rep: Dear Adrian, sorry for the late reply. We are sorry, but we do not offer single TJ07 window kits. We only sell the TJ07 with window already integrated. The problem or special with the TJ07 is, that it use a unibody structure, as you maybe know. The special is, that this unibody moves a little bit after production. Therefore we need to put the sidepanel at the case directly during this process, so that the sidepanel can also squeeze or move with the unibody panel. If we send you a single new panel, it will propably do not fit and you cannot install or take out the sidepanel very easy. We do not suggest this solution. A better solution will be, if you let your TJ07 panel upgrade with a window kit, by a qualified modder. Sorry that we cannot offer you another solution. Mit freundlichen Grüssen / Best Regards Stefan Galow