Since Microsoft no longer sells this version directly, you have to look at third-party sellers:
Check local sites or thrift stores for "new old stock" boxes, though these are becoming rare. Critical Considerations where to buy office 2013 cheap
Extremely cheap keys ($5–$15) found on some sites are often "Volume License" keys intended for large companies. These may work initially but can be deactivated by Microsoft later if they weren't meant for individual sale. MICROSOFT OFFICE 2013 PROFESSIONAL PLUS - Wiresoft Since Microsoft no longer sells this version directly,
Because it no longer receives security patches, your computer is more vulnerable to malware that targets older Office vulnerabilities. MICROSOFT OFFICE 2013 PROFESSIONAL PLUS - Wiresoft Because
Specialist sites like LicenceDeals or Wiresoft often have digital licenses available for significantly lower prices.
You can find both new and used physical copies (DVD/Product Key Cards) on sites like eBay.
While you can still find it at various retailers, there are important risks and better modern alternatives to consider. Where to Buy Office 2013 Today