I'd suggest looking at what's available from the RC hobby market.
Andymark has put together a kit of low cost components that would likely get the job done:
http://www.andymark.com/product-p/am-2493.htm
It probably includes a bunch of stuff you don't need (e.g. battery)
Alternatively there are lots of online retailers in the RC space. Hobby king is a popular cheap retailer.
You're going to need a controller (radio) and a receiver that are compatible. The receiver needs to have enough channels to drive all your PWM devices (motor controllers). You may also need a BEC (battery elimination circuit - just a voltage regulator) to provide power to the receiver if you dont have a 5v source on the robot already.
http://www.hobbyking.com/hobbyking/s...e_systems.html
Range should be good enough for a football field for pretty much all the options available as you're operating line of sight, bu read reviews to convince yourself before you buy.