Welcome to the forums :)
What MIDI Controller do you have?
I don't know what controller Jim uses, but any MIDI footswitches, pedals, and infact almost all USB/MIDI controllers with knobs, buttons, faders, can all be used with Mainstage... as Mainstage allows the user to map a virtual knob, fader etc that you make to a physical one on a MIDI Controller.
There are many to choose from with different features and within a variety of budgets too.
A good place to start would be to think about whether you want a MIDI keyboard & controls, and if so how many keys... or just a controller with knobs/faders/buttons etc.
Also, what would you like the controller to..er..control? e.g. bypassing a plug-in would be better to map to a button. Controlling the level of the gtr channel strip would go well with a fader. Adjusting the pan would map well to a knob... these are just suggestions...
How do you wish to control Mainstage? With your hands, feet, or both?
Lastly, what is your budget?
There are plenty of control and Keyboard surfaces available you might wish to check out in the meantime: