Home Depot has some for about $15 for a 10 ft roll.
The Bitchathene definitely helps to keep things dry, but I got an ole Jefe that is busting apart at the seams. Everyday I boat I get more cracks.
I have committed to finishing out the season on it so I bought this cheap plastic welder $60 which I think works pretty good and it is easy to use. It also as a small amount of metal mesh for those trouble spots.
It is different from a heat gun as it has a small tip you put right on the plastic as opposed to blowing hot air.
check it out Urethane Pro Kayak & Canoe Welder w/ Green Rods at Arizona Tools