I also found what I will assume is the 6th serum (I skipped ahead a little). When you get to Silent Hill, before going into the prison go to the end and you will enter a boiler room. Come out the other side and go to Alchemilla Hospital and it is sitting on a bench right in front of the entrance.
I found the 5th serum in SH Homecoming. After you come up from the underground area with the ceremonial dagger you will be back in the cemetery. Open the tomb directly across from where you exit in the founders garden with the dagger. Inside on the right is the serum.