If you are talking about the stuff between the edge of the vent glass and the metal channel that holds it and swivels then what you do is go to any glass place (I went to Binswanger Glass) and show them what your talking about. it's not exactly rubber but some kind of flexible glass sealing material though. they gave me a small roll of it to do both of my vent windows and then extra on top of that. He let me have it for free since it's not something that people normally ask for
but i'm sure you wouldn't pay more than a couple bucks for it at other places.