98sub2500leg

I was just reading a thread on leaking valve cover gaskets. I have a 97 outback limited that I'm replacing the valve cover gaskets on. I put on aftermarket ones when I just did the heads and they are leaking. What brand of gaskets did you use? The biggest question for me is where you got the little black plastic half moon plug at the rear of the head. One of mine got broke probably by the machine shop and I need a new one.