Hatchet

Sampson and his son Ainsley are fishing in a swamp. While Ainsley is urinating, Sampson falls silent; Ainsley finds Sampson killed. He grabs his harpoon, only to be murdered by a monstrous being.
During a Mardi Gras celebration in New Orleans, a group of friends including Ben and his best friend Marcus decide to go on a haunted swamp tour. They find the tour closed because the guide, Rev. Zombie, was sued for negligence. Rev. Zombie suggests they try a place farther down the street, owned by the over-the-top, inexperienced tour guide Shawn. Marcus decides to leave but changes his mind upon seeing two topless girls: Misty, a ditzy porn star, and Jenna, a bossy, boastful, up-and-coming actress. Their sleazy director, Doug Shapiro, is also present. Ben pays for himself and Marcus and Shawn leads them to his tour bus, where the other tourists, Jim and Shannon Permatteo, a Minnesota married couple, and the quiet, hot-tempered Marybeth are waiting.
Shawn does not know what he is doing, which the others realize as they arrive at the swamp. Shapiro has Misty and Jenna strip down and film a scene for Bayou Beavers as everyone boards the boat, while a homeless swamp-dweller named Jack Cracker warns them away from the swamp. Shawn leads them through swamplands and past abandoned houses, including one where Victor Crowley, a deformed creature, lived. The boat hits a rock and begins sinking, leaving them stranded.
As the crew walks through the woods, they encounter the shabby Crowley house, and Marybeth shares the legend of Victor Crowley. Victor was a deformed child with a rare disease, bullied by other kids and was kept hidden by his father, Thomas Crowley. One night a group of mean teenagers stumbled across the house and threw fireworks at it to scare Victor. The house began burning, and Thomas returned, causing the teens to flee. Victor was killed when Thomas accidentally hit him in the face with a hatchet while trying to break down the door. Marybeth claims that Victor roams the swamp at night, looking for his father. Marybeth then explains that they are not safe in the woods, but the crew do not believe her.
As Jim and Shannon approach the house, Victor comes out of it and kills them, causing the group to flee. Marybeth shoots Victor with a handgun, but he gets up and resumes his pursuit. Shapiro splits from the group and is killed by Victor. The remaining survivors decide to return to the house where they can arm themselves.
While at the house, Marybeth and Ben discover her brother and father's remains. Marcus, Shawn, Misty and Jenna hear a noise in a bush. Marcus goes to investigate and discovers that it was only a racoon. Victor then surprises the group and gets a hold of Jenna, then shreds her mouth off with a belt sander. Marybeth and Ben return and attack Victor. While the other survivors flee, Shawn tries fighting Victor, but instead is killed. Victor then kills Jenna.
The survivors decide to lure Victor back to his house and set him on fire with the gasoline tanks in the shed. Ben goes into the shed to retrieve a gasoline tank while Misty stands guard and Marybeth and Marcus act as bait. Marybeth and Marcus discover that Misty is missing, and her corpse is thrown onto Ben by Victor. Ben finds a tank and throws it on Victor while Marybeth and Marcus set him on fire, but he is extinguished due to rain. They start running away, but Marcus is grabbed by Victor and killed. Victor grabs a gate pole and chases Ben and Marybeth, throwing it into Ben's foot. Marybeth bends the pole until it is pointed at Victor, who impales himself upon it and collapses, apparently killing him. Ben and Marybeth board Sampson's boat and flee; Marybeth is snared by seaweed and pulled underwater. She sees Ben's arm sticking into the water for her to grab, but is pulled up by Victor, who is holding a dying Ben's severed forearm. She screams in horror, as Crowley roars into her face, setting the opening for the next film.

Question: Where do they get the gas tanks?
Answer: the shed