@Bruce-Gray Hello Bruce and welcome to the forum,
you probably have already figured out, that devices that have a port are listed at:
https://devices.ubuntu-touch.io/
The M10 3rd Gen is not listed there and it will most likely not be compatible with the port for the 2nd Gen device, because it has very different internals (different chipset). So you should not buy the 3rd Gen device for using it with Ubuntu Touch.
I could find the following (probably overpriced) 2nd gen device on eBay:
https://www.ebay.com/itm/115848957812?_skw=Lenovo+M10+2+gen+4gb&epid=21045777728&itmmeta
But if you want a more powerful device, the Volla Tablet will probably get Ubuntu Touch support in the coming weeks:
https://www.indiegogo.com/projects/volla-tablet-simplify-your-digital-life#/
Volla has already made multiple devices with Ubuntu Touch support.
Please be aware, that Ubuntu Touch is not a "normal" Linux distro (more restricted, some desktop apps do not work) and depending on your usecase, it is maybe better to buy an Intel or AMD tablet and use standard Ubuntu or Debian (I think it is fairly easy to install Linux on some Surface Tablets).