you probably won't find it. it most likely was black tar that used to be sold in metal containers. as a grandfather myself i remember this from my youth. you can use hot water compresses for the same effect. there is an ointment you can get from your pharmacist. it looks like the black tar but has a different name. it is kept behind the counter and works wonders. the black tar salve that you are talking about is called ichthammol ointment 20%. walgreen pharmacy has it. it works wonders.
my grandmother used what she called black salve to treat boils. i can't seem to find it anywhere it was a drawing salve