You can't get a Cadillac Escalade *specifically* in Grand Theft Auto IV. There isn't a vehicle in the game that's officially branded as a Cadillac Escalade. GTA IV uses fictional car models based on real-world vehicles, but doesn't include direct licensed replicas of specific makes and models like the Escalade.
The closest you might get would be finding a large SUV that resembles the Escalade in terms of size and shape. However, there's no single perfect equivalent. You'll have to settle for something similar visually.
