Check ebay. I recently bought a 2013 Mac Pro off ebay for a good price. My 2009 Mac Pro was no longer supported. I figure the 2013s will probably have another 5 years after whatever new model they come out with next year.
Ya, I couldn't get it to work and I have more money than patience. I also didn't buy one when they first came out, so I was happy with my 4 years of holding off for a good price. Besides, new (to me) hardware always motivates me to build products. I'm working on an Azure app on a Windows 10 VM inside a macOS. :) I need to get around to learning how to work on C# apps in macOS rather than Visual Studio, but I haven't quite gotten around to it yet.
Either way, now I have a trashcan / hand warmer on my desk with a TB connected external drive cage with 2 mirror sets, and it makes me happy.
Can’t quite remember the details but I think it’s even possible to bring it up to 2012 spec which will be supported in Mojave.