I don't think Microsoft sells them any more (but why not call them?). In that case you would be limited to the usual second hand sites.
Be aware that Home, Pro and Media Center don't accept each others key, and that versions with different service packs might have different keys also.
I am unable to find my XP disc. However I have that original sticker on the case (now running Ubuntu) that has the activation number of the OEM disc.
Is there a way to get an xp disc from the Microsoft on the bases of the key number?
I have tried to acquire a disc from my children and grand children to load XP and then activate from the key I already I have, but failed because all have moved up from XP.
Be very grateful for help.