Its under the car at the rear near the tank.
A P0442 code is for a small leak detected in the evap system.I wouldnt just go and throw a canister on it until i had it checked out.
Any number of things can cause this.
Who told you it needed a canister?
Check the gas cap.If left loose or off,it could set this code as could refueling the vehicle while its running.
I would first check the cap,make sure its on correctly,clear the code and see if it resets,if it does,have it checked,if not,you saved some cash.
Click the link below,give it a read,explains the EVAP system operation..