Sculpturesupply.com has a lot of cool stuff for sculpting and artistic hobbies, but scroll down and you see a little, tiny "Makerbot 3D" section that sells pretty reasonably priced Kapton and ABC (I got a roll of Nuclear Green

Less than a week order to receipt.
I am not affiliated with this company.. just had a hard time finding reasonable price wide Kapton locally (the roll I got from them is 4.75 inches (120mm))