Chinmoy, we (LCN) have S6a/S6d implementation. It is a commercial product. But we offer evaluation license, so you will be able to use it for 1-2 months without buying license, for testing.
Our focus is Diameter and interfaces, and we offer a sample client and server (with C code). But client just sends dummy requests (syntactically complete and correct) and server sends a response. If you are looking for serious MME/HSS simulation, we don't have it. Interface is complete, though.
If this is of interest, let me know.