Hi, the latest NSubstitute releases are no longer stand-alone (they require the download of dependencies which are managed by the new MSBuild system) so it doesn't really make sense to offer them as a separate download any more.
The older releases can still be downloaded from the website github page*. These downloads include all the required dependencies ilmerged into a single DLL:
The v2.0.3 release is a very solid release so you should be fine to stick with that for Unity3d. :)
Regards,
David
* If you're really keen on the 3.x releases you can download the nupkg from NuGet and extract it as a standard ZIP file and grab the DLL from there. You'll also need to download and reference dependencies like Castle.Core, and probably System.Threading.Tasks.Extensions as well.