Heat what, the deadener? I think you all are nuts on this one. First, I wouldn't even use a heatgun, let alone go and buy one. It's not going to heat evenly, and that's going to be annoying. It doesn't need to be that hot. You can leave it inside until you're ready to use it, and that will be good enough to make it pliable.
If you really wanted to heat it: You can put it over the oven, use a hairdryer, put it on top of the dryer, hang it near an air vent. If you have a water heater cabinet, or interior furnace you could lay it next to. You could set it near an iron, put it in the bathroom with you, an electric blanket, heat a baking sheet and lay it on that, lay it on a warm hood.