OS that supports 32 bit software

What is the latest version of Mac OS that supports 32 bit software, and what site would be the best (including the URL) to download it from? I have a piece of scientific equipment I need to use so I am going to load that OS on an external hard drive and use that as my startup disc when I am in need of this particular (and very expensive) piece of equipment.

High Sierra. When it comes out, Mojave will be the last MacOS to support 32 bit apps.

And download High Sierra from MAS using this URL <https://itunes.apple.com/us/app/macos-high-sierra/id1246284741?mt=12>, although at this moment it will only give you 10.13.5. Apple has been a bit slow to make the current version available to all.