Do you already have the product key you just need the files?
Good question, if you have a license just go to office.com, you insert your install serial and you'll get a download link.
You can also get 2013/2016 cheapie licenses off eBay.co.uk for R50 odd, they email you the serial and download Iso link quickly/or just download from office.com, they install and activate online 100% but somethings probably not kosher.
I looked around on Google and people seem to think they are Msdn keys, the ebay listings imply oem licenses with scrap towers(Eu citizens are allowed to sell oem apparently if it's with scrap) and sellers have high ratings and have sold thousands. Search for scrap oem 2013/2016 office. Make sure you sort by cheapest because as with everything the clueless get ripped off, when I last looked there was a seller selling them for R800, had sold 50 already, identical thing was also selling a page up for R50.