You need to be an authorised Microsoft refurbisher to purchase the refurbisher COA labels. The labels come without a Windows licence key, just a serial number. You log into your refurbisher account and use the original (eg XP, Vista) and new COA serial numbers to authorise and generate a Windows key. If you were to acquire a refurbisher licence any other way, I suspect it would violate the licensing conditions.
I'd recommend joining the refurbisher program instead. Joined it myself quite recently. It's fairly easy to get approved.