Previously, Microsoft partners could access Office 2013 through Action Pack. Those downloads have ceased, but if you were a partner in 2015, you likely have a local copy.
Since Office 2013 is discontinued, Microsoft no longer sells it directly. Legitimate options include: Office 2013 Home And Business 64 Bit Iso Download
Microsoft’s official download page for older Office versions (login required):
https://account.microsoft.com/services Office 2013 Home And Business 64 Bit Iso Download
If your product key is valid, the site will offer the correct ISO version (32-bit or 64-bit) for download. Office 2013 Home And Business 64 Bit Iso Download
Microsoft does not publicly host direct download links for older ISO files indefinitely, but there is an official method to retrieve the setup files using your valid product key.