Are you getting this error on a Build Server (like Azure DevOps or GitHub Actions)? The build agent might be missing the SDK.
While you shouldn't install it manually in a .csproj file for a local app, you can force the download for a specific environment by referencing the package directory.
However, the correct approach for CI/CD is usually to ensure the build agent image includes the Windows 10 SDK (10.0.17763.0 or later).
If you absolutely must download the .nupkg file directly (perhaps to host it in a local feed), the official source is the NuGet gallery:
You can download the .nupkg directly from NuGet.org by clicking "Download package" on the left sidebar of the package page.
Prerequisites:
Steps to install:
Symptoms: Warning messages stating the framework version (2.2) doesn't match the compiler version (2.1 or 2.3). Solution:
.dll files (like mrt100.dll) from the internet. Always install the full signed package container.