It does exist on NuGet, and was last published 2013-02-08, so I don't understand why you couldn't find it. Try searching for System.IdentityModel.Tokens.ValidatingIssuerNameRegistry. The best way is to install it is using the NuGet packet manager.
You can find the package here: https://www.nuget.org/packages/System.IdentityModel.Tokens.ValidatingIssuerNameRegistry/4.5.0