Go to any auto body supply store, and ask for the stuff used to install older windshields, or, order it off Ebay. Guns is correct in that is it butyl, but you want a bead, not a strip. The stuff shown in the following link is exactly what you need:
Auto Glass Sealant Adhesive Butyl Tape 15' Roll Soft Seal 1 4" Round | eBay
1/4" round is the size you want. 3/8" round is okay if that is all you can get. Seems to me a 10' roll does one door with some left over. I am not sure if a 15' roll will do 2 doors, but it would be close. Don't be afraid to piece together left over pieces ... this stuff sticks to itself (and anything else) very well.
2001 Bonneville SSEi (retired at 365,000 Kms.)
2014 Jeep Grand Cherokee Overland (yes, I know its not a GM)