I suppose that you can't get it from the manufacturer. That would be ideal, but then it would be too easy for you to remove all of their adware.

Any MSDN subscriber can download a copy. I would not get one from an untrusted source such as a peer-to-peer system. But note that the OEM installs often take different keys; your key may not work at all on a straight Microsoft install disk. I've seen it before.