Looking at the style of door seals on the gmt400 looks like common door edge seals with side bulb sealing portion on it. I tried ordering a roll on Amazon, the profile and shape is correct, but its a little bit smaller in every dimension. So I'd think it has to be available cheap in 30ft rolls or bigger. My trucks a K2500 suburban with a tailgate and the rear seal on the body looks like the same stuff too.