|As XP is now 10 years old (happy birthday XP!) and Vista is nearly 5 years old, the only retail copies you'll find will be old stock & so will be getting more scarce as days go by. I (and you) can get a free,'legal', copy of XP home by going down to a local tip and removing the COA sticker from the sides of discarded computers that people have dumped & using a generic oem disk to install it - Vista will be going the same way soon as well.|
As soon as Microsoft start selling a new OS (Win7 in this case) then previous versions essentially become obsolete and retailers will only stock them for a short space of time - that's why you won't find many, if any, copies on-line.
Edit: Of course I am posting this from the throw-away society that is the western world, so I apologise if your waste tips don't have the sort of discarded systems that we have here, though the way our economies are currently going we may soon all be in the same situation, so I'm stockpiling any COAs that I find ;-)
"I've always been mad, I know I've been mad, like the most of us..." Pink Floyd