Electronic Speed Controls (ESC's) used in hobby remote control (RC) cars are often used to control 540 size motors, costing between $40-$100. They accept the same PWM signal that's used to control servos. You should be able to replace the continuous rotation servo with this and simply adjust some constants in the code to account for a different neutral, full forward, and full reverse pulse widths.
A cheaper and more DIY solution would be to get H-bridges (try L293D, ~$2) and gear motors (~$8 solarbotics ones) to effectively create your own "continous rotation servo". This would require some change in the code (no more PWM to control motors, must generate H-bridge signals instead).