You will not find portmsys.sys
anywhere to download. It is actually part of portmon.exe
The way that many Sysinternals tools work (Process Explorer and Process Monitor) work, is that they bring their own driver embedded into the main executable. When the program is started it extracts the driver file from itself and installs it and then uses it to do its thing.
portmsys.sys
is the driver for Portmon, and on 64-bit Windows it extracts it from itself, but it is unable to install it.
I think you have to live with the fact that it doesn't work on 64-bit Windows.