Where do you live? I'm in the US and use US sources; sounds like you are in England. I'll provide a link to the wonderful list of international sources.
But there are really four possibilities. In increasing order of cost: Black market (cracked, torrented, whatever) which are illegal and likely pre-loaded with malware.
Grey market, like an OEM disk provided to or by a manufacturer of PCs that may or may not work with your hardware. Sometimes it's hard to tell grey from white on eBay. There are perfectly good unused licenses there, but I wouldn't go for anything other than Full Retail in original shrink wrap.
White market, here in the US old stock at Staples, Microcenter, or the ever-reliable Newegg.com, but now we're...