Most all of the usual suspects sell the vent window rubbers. Installing the rubber, is a rather invasive process, when done properly, to emulate the quality, fit, finish & function of the unmolested original equipment.
Steele Rubber Products is well-known for their reproduction rubber items and your experience is an unusual occurance. Have you discussed the experience with Steele? I would be surprised if they didn't make every effort to course-correct the issue.
You can also try:
www.lmctrucks.comwww.classicparts.comwww.bowtiebits.comwww.oldchevytrucks.comwww.truckandcarshop.comGood luck and keep us posted on the outcome.
