When you install a nuget package in your project then Visual studio cached it for you for future purpose. If you live offline few time you can add the this cache directory as your nuget source.
For see the folder where nuget cached then just go to
Tools > options > Package manager > source > Cached packages > browse. You can add this cached folder as source.
For use nuget package without Visual studio you can try this trick.
open this in your explorer
Now rename the file as described here.
for example bootstrap.3.0.2.nupkg to bootstrap.3.0.2.rar
open the rar and see the content folder. this folder contain the assembly/library that nuget install in your project.