Microsoft has made available the relevant assemblies as packages on NuGet now.
System.ServiceModel.Primitives is the base package; add the others if necessary to your project.
Update (April 28th, 2022):
WCF has been partially ported to .NET 5+ with an official library called CoreWCF: https://devblogs.microsoft.com/dotnet/corewcf-v1-released/