I used to work at a glass company and having just taken my inside door panel off, the piece you need is a common strip of auto window molding. Call up the glass shops in your area and ask them if they go through a company called C.R. Lawrence. They should definitely be able to get you what you need. You may just need to bring in a sample of the cross section of the material. The C.R. Lawrence catalog stocked this as of several years ago, and it's extremely likely they still do.