SampSharp hosts it own NuGet package repository which contains the framework and various other SampSharp related packages.
To add this repository to your Visual Studio's Package Manager, follow these instructions:
Tools > Options...
NuGet Package Manager > Package Sources
If you have made a useful extension or library for SampSharp, please contact ikkentim via Gitter or the forums! We're happy to create an account for you on the NuGet repository so you can upload and maintain your own packages.