Ian, don't you have a radio shack nearby. They have heat sink compound but it will be a tiny tube.
I bet any decent sized electric supply house carries it too if they deal with industrial customers. You need that compound on the SSRs you see in solid state controllers.
There are plenty of places online to buy the big tubes and any real electronics store should have it. Usually a medium sized city can only support one of these since nobody fixes anything anymore.
