Any auto parts store should have it. You'll have to take the door panel off, but after that its pretty much a remove and replace affair.

Do yourself a favor though, pull the motor, and you'll see it has a little gearbox on the end. Take that cover off and see if the gears are stripped. Alot of times, the gears are stripped and the motor is fine. You can buy the gears at the auto parts store as well. Alot cheaper than a motor and probably more reliable (remaned motors are a bit iffy)