MTH DCS 50-1003

You need to get 50-1001, the complete DCS system.

50-1002 is just the handheld

50-1003 is just the TIU

50-1001 is both

You'll also need a transformer/power supply.  I recommend the Lionel 6-22983 (180 Watt Powerhouse Transformer Power Supply) and the connecting cable (not sure what the part number is).

TT,

Although you do not need a separate power supply for the TIU, it is recomended to have one.  If you do get one, the Lionel one that Bob suggested is massive overkill. All that you need is anything that provides at least an amp and half at 12-20 volts AC or DC, and that has a barrel jack connector that fits the TIU's Aux. Power port.

Any one of the MTH "Z-bricks" will work, as will a  1 amp (yes, it will work even hough it's rated at 1 amp) power supply from Radio Shack. The item number escapes me at the moment, however, it has interchangeable plug tips and the plug tip that you need is "Adaptaplug M".

I have a Legacy Remote Control hooked up already to 3 separate track runs with a TPC 300 hooked to each track run.  Will I need to also purchase a separate Power Source for the TIU?

You can wire the TIU in between your TPC 300' s and the track. Then all you need is the small Radio Shack unit Barry mentioned.