How can I download Microsoft office version for my old Mac that is working fine

Go to Microsoft's website where you can download previous versions of Office. You will probably want Office 2011 or 2016. If you prefer to use Office 2019 or Office 365, then you will need to upgrade macOS to Catalina: How to upgrade to macOS Catalina.

Last year I had trouble getting old versions of Office 2011 to become authorized (server issues) after performing a clean install for several clients.

You can download and install the free open source office suite LibreOffice. Sometimes you will encounter formatting issues when opening documents made with Office, but these issues can be minimized if you use a document template to keep the document formatting contained at the beginning of the document (makes it much easier to fix formatting issues).

If you need MS Office, then you could buy a subscription to Office 365 which is web based, but you will need to use a third party browser such as Firefox, Google Chrome, or Vivaldi since the Yosemite version of Safari is outdated.

If your Mac can support a later version of macOS, then you could update to the latest supported version and by an actual copy of Office instead of the Office 365 subscription.

Edit: I should add there is no way to know how long these third party browsers will continue support of Yosemite. I really don't expect them to support Yosemite for much longer.