Some online marketplaces still list "new" keys. However, proceed with extreme caution; these are often "grey market" keys that may be blocked by Microsoft or contain malware if bundled with downloaders.
Sometimes, simply entering the 25-character key in the system properties works. Windows 7 Professional Key
Many older laptops and desktops have a physical Certificate of Authenticity (COA) sticker on the underside or inside the battery compartment. Some online marketplaces still list "new" keys