An industrial supply house should have it. I've even seen it at my local small town industrial sply. hse. I keep several tubes of the heavy and light bodied stuff. Dow. These tubes are the size of a large toothpaste tube.
McMaster Carr, Grainger i'll bet offer it.
One place I know has it is called Plastic Process Equipment. They are plastic injection moulding supply specialists and I know they sell it. That's where I used to get it.