most likely
www.gmpartsdirect.com
or somekind of gmgoodwrench site. Don't remember exact address though
just a sidenote too I thought I'd throw in to be helpful - If anybody has problems with their weatherstripping, here are a few tips:
Clean it good one day with water, and then armorall it after it's dry. This will keep the rubber from drying out and cracking more.
In the winter, use a light mist of silicon spray on it. It will keep it from freezing together when it gets really cold out, and will also help keep it from cracking.
Hope that's helpful.