I currently have no Windows system on any PC. I have Ubuntu. I want to
purchase and download a Win10 bootable install iso for use on another PC
(an old Dell Inspiron N4110 laptop). I can make a bootable USB stick
using linux command line something like:

dd if=windowsinstall.iso of=/dev/sdb

I know how to do that.

What i need to know is where to purchase and download the iso file that
will boot the laptop into the Windows 10 installation routine. I assume
I will get a long registration code when I purchase the file that I will
need to enter during installation. I prefer to get the file from
Microsoft if possible. TIA. Bill S.