You may find unused physical copies (which include the key on a sticker inside the box) on sites like Amazon or eBay. Be cautious of "key-only" digital sales from unknown sites, as these are often unauthorized or non-functional.

Since official sales ended years ago, you must rely on secondary markets or your own records:

If you bought it digitally years ago, search your inbox for "Microsoft Office" or "product key" from the time of purchase. Recovering a Lost Key