There is a Hungarian joint on Ebay selling pretty inexpensive controllers for brush motors. Here is one of their units that may work for you :
150A-10-40V-12V-24V-current-limited-DC-motor-speed-controller-PWM-RS232-arduino | eBay
Find many great new & used options and get the best deals for 150A-10-40V-12V-24V-current-limited-DC-motor-speed-controller-PWM-RS232-arduino at the best online prices at eBay! Free shipping for many products!