You need to say how much current your motors need.
The Toshiba TB6612FNG is a nice cheap $9 - $10, and small, dual motor driver if you are in the 1Amp continuous, 3 Amp peak, kinda range.
Available from SparkFun and others.
So choose your motors - based on robot weight, speed/torque required - using Admins calculators.
Then find a controller that can drive the motors.