The rubber at the tip of my radiator cap (not at the underside of the cap, but at the far end, the part that goes into reservoir first) is old and brittle.

I ordered new rubber from a place, but ended up getting a part for the underside of the cap, which I don't need. I asked about rubber for the part I do need, and was told they don't make that, and I might have to buy a new cap. It's sort of insulting to spend $30 on a new cap, when what I really need, is a 50 cent piece of rubber. Is there any place where I can buy radiator quality rubber, and simply replace?

jeff 1991 300d, 167k