It was and might right now still be possible to subscribe to Office365 and chose deliberately to install the previous i.e. Office 2011 version. However I did relatively recently see an announcement that this was going to be stopped meaning you would then only be able to use Office 2016. The impression I got was that if you had downloaded and activated Office 2011 prior to this deadline it would still work but after this deadline you would no longer be able to download the previous version.
Another approach which like Office365 is probably not any help to you is an Office Volume License. A volume license is typically for a minimum of five copies and historically has also allowed you to install the previous version. A volume license also covers both Mac and Windows versions. So in theory you could buy a volume license and then download and install Office 2011 for Mac.
You final option is to find a reseller still selling Office 2011. The problem here is that I would suspect such sellers would tend to be less reputable. As a result the product might be fake i.e. using a fake or hacked key, or might be a used key which means Microsoft might refuse to 'activate' it again. Amazon and/or eBay would be two typical channels.
Note: Even if listed on Amazon it is likely to be from a 'Marketplace Reseller' rather than Amazon themselves. While not all Marketplace resellers are dodgy a lot are.