definitely check out MST (Max Speed Technology). They have several different "levels" of dedicated drift chassis and RTR's available. Even the most basic (cheapest) MS-01D is a good drift chassis.
if you're looking to go cheap, and you can get your hands a good used HPI Sprint 2 chassis, I know a lot of drifters have started out with the S2, S2 Drift or S2 Flux. it's not a dedicated drift chassis, but swap in some drift tires, adjust the camber and switch out the suspension and it's good for learning to drift with.
also, you can look at Yokomo. they also have some dedicated (and non) drift chassis that people like and have lots of option parts available for.
no matter what chassis you choose, when it comes down to it, the most important thing is how the car is set up. the right compound drift tires and dampers/oil/springs for your style and surface can go a loooooong way.