Project X

In Pasadena, high school students Costa and J.B. plan to throw a party for their friend Thomas' birthday, aiming to increase their popularity. Thomas' parents go away for the weekend, leaving him alone in the house, but warn him not to have too many people over, and not to drive his father's Mercedes. Thomas is reluctant to have the party at his house, but Costa continues with his plan. Costa hires AV-student Dax to chronicle the night's events. Costa and J.B. advertise the party throughout the school. Thomas himself invites Alexis, the hottest girl at school, and his best friend Kirby, who has a crush on Thomas.
Costa, Thomas, and J.B. go to buy marijuana from drug-dealer T-Rick. While T-Rick gets the marijuana, Costa steals his lawn gnome to use as a party mascot. As they leave, T-Rick discovers his gnome stolen and chases after them but they escape in Thomas' mini-van. As night falls, the party's start time passes but no one turns up and Thomas worries that the party will fail. Suddenly, party-goers arrive en-masse. Thomas intends to limit the party to the backyard and poolhouse with the house guarded by two young security guards, Everett and Tyler. However, more and more people begin to arrive and the party moves beyond their control and into the house. Thomas questions Costa on how he advertised the party, forcing him to confess that he put ads on Craigslist and on a local radio station, worried that no one would attend.
Things escalate quickly and the police arrive, responding to a noise complaint. The partygoers remain silent, convincing the officers that the party has already ended. The police leave and the party resumes. T-Rick's gnome is smashed, revealing that it contains a large amount of ecstasy tablets, which are quickly consumed by the partygoers. Thomas kisses Kirby and reveals that he loves her. Alexis meanwhile flirts with Thomas throughout the night and eventually seduces him. Kirby walks in on the pair as they are about to have sex and leaves the party upset. The noise and chaos of the party, which has now spilled into the surrounding neighborhood, receives televised news coverage with helicopters flying over the house. A dwarf guest drives Thomas' dad's Mercedes into the pool after being put into an oven by other partygoers.
The police return in force but are overwhelmed and repelled by the guests. They decide to let the party end naturally before moving in. T-Rick arrives armed with a flamethrower in an attempt to reclaim his gnome. He begins setting fire to the trees and cars in search of Costa, forcing the guests to flee and the party to end. The police shoot his flamethrower pack and it explodes. Thomas, Costa, J.B., and Dax flee with the other guests as Thomas' house burns and the SWAT team moves in to retake the neighborhood. The neighborhood is left aflame.
By morning, the friends return to their respective homes to discover what punishment awaits them. After his parents return, Thomas' father commends him for managing to throw the party because he thought he was a loser, but he uses Thomas' college fund to pay for the damages. At school, Thomas, Costa and J.B. are cheered by the students and Thomas reconciles romantically with Kirby. In the epilogue, T-Rick is recovered alive following the explosion, Thomas is convicted for disturbing the peace, contributing to the delinquency of minors, and inciting a riot, and Costa and J.B. are acquitted; Costa because of his expensive lawyer and J.B. because his parents convince the court that he is mentally incapable and unfit to stand trial. Costa, however, is currently waiting for the results of three paternity tests. Dax, meanwhile, is under investigation for the disappearance of his parents. In an interview with Jillian Reynolds, Costa promises his next party will be even better.

Question: What do Costa, Thomas, and J.B. go to buy from T-Rick?
Answer: marijuana