I had a cracked and deteriorated o-ring seal that goes around the metal plumbing vent cover in the driver's side of the van (see picture below). I found a great source on Amazon for this seal. It comes in a package of 5, so I have a few left over. It worked as a perfect replacement for me. If you need one, send me a PM and I can see about sending one to you in the mail. See the link below if you want to order some for yourself.
Picture of the plumbing vent I'm referring to: https://photos.app.goo.gl/UmuwymSRMiSQgZKRA
Source on Amazon: https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1