Question about getting a rack..

Lets say i get a 3 piece rack.. how do you guys heat up your racks? ive seen some pictures and ive never seen any heat source in any of them

Well you don't see the heat source. In most racks it's flexwatt and it's either put in by the manufacturer of the rack as either back heat (run along the back inside of the rack) or belly heat (run along each rack shelf inside the rack). It's controlled by a t-stat so it gives stable, controlled heat.

Heat Cable or Heat Tape (Flexwatt) controlled by a thermostat of your choice (Herpstat, Helix, etc..)

As Jo said, the heat items are usually included in the rack, but you have to buy a thermostat to control the temps.