How to validate Outlook add-in when required platforms are unavailable

We are developing an add-in for Outlook. Upon submission to the Office Store, it was rejected:

"Your add-in is not working in the Outlook 2013 client on Windows 7 with Internet Explorer 11."

Fair enough, however getting an authentic copy of Outlook 2013 and Windows 7 seems to be a matter of being lucky enough to have purchased these products at some point in the past. We were fortunate that we had a Windows 7 laptop specifically for testing against Windows 7, but Office 365 just dropped licensing for Outlook 2013 in Feb 2017, so the installed client does not function. We are unable to troubleshoot.

Short of buying old stock of Outlook/Office 2013 that is not linked to Office 365, are there options for acquiring older software from Microsoft to test apps heading to the Office Store?