Since Office 2013 is discontinued, Microsoft no longer sells it directly. Legitimate options include:
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. Since Office 2013 is discontinued, Microsoft no longer
Websites like Archive.org, Heidoc (HeiDoc.net), and certain tech forums have preserved the original Office 2013 ISOs. Here is how to navigate this safely: Known good SHA-1 hash for Office 2013 Home
Known good SHA-1 hash for Office 2013 Home and Business 64-bit (English): You should verify this against any file you download. If the hash does not match, delete the file immediately—it may contain malware.
Once you have the legitimate Office2013_HB_64Bit.iso file:
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.