There are two places near me that will have that stuff, when I got the cut to fit for my truck it was $30 for a PAIR. Checkout "Classic Industries" and "truck and car shop"
Outside of that, Rock Auto Carries cut to fit for like $14 each, I THINK the part number you want GM1390101 but verify. They are known as door seals, door weatherstripping, cab weatherstripping and any other combination of those words.