To complete the task you need two propane tanks. They are given out by Skier. If you lose them, find or purchase yourself.
The first place is located on Woods, namely in the Scav bunker, where The Goons are usually hang out.
Find the stashing location on Woods


The second place is located on Customs, namely at the crossroads near the river. You need to go inside the small store and knock down the door to the warehouse.
Find the stashing location on Customs