I used cork in mine, but they have a neoprene one as well. The key either way is tighten and make sure things are "square", but do not over tighten.
In a pinch I have used duct tape to limp home.
They are available from most chandleries here in the US, not sure about there. You will need the diameter of the globe and then the width of the actual gasket.
Best of luck