The good news is that you may already have it… dig in your old drives, or maybe you have it in your GoG library, all you need is a basic installation of it with a cdkey. The bad news is that it is very hard to find now that GoG replaced it with EE edition.
We can help you on Discord: https://discord.gg/pM3ja9n
You have 2 options:
a) Download the latest game data (bigger download but recommended):
b) Download the latest binaries and SinfarX (included in the download above):
Then Extract the file that you downloaded above in your NWN installation folder.