Why not design your own shield? I mean what do you need the motors to do? Do you need speed control? Do you need directional control? Whats the maximum current that runs through the motors?
I'm doing a similar project where I need to control 2 12V DC motors that need speed and directional control. All you need is Power transistors and H-Bridges that can handle your max current and you should be good to go. Maybe check some things like voltages drops, admittedly I kind of dropped the ball on that with my project, but like The maximum current on my motors are 4A, but that's only around stall, they typically run around 1-2A, so I have have a TIP120 and an L298 in series for each motor. The power transistor is used as a switch with a PWM value from the arduino on the base. The L298 is actually a dual H-bridge but you can tie em together to act like a single 4 A H-Bridge, and that gives me directional control with digital inputs from the arduino. In my case, the L298 was kind of a poor choice because of the voltage drops and how they propagate when you do what I'm doing with them, but it still works. Best part: You can get all the stuff from Jameco for like $.50-3.00 a part. You will have to make a board, either design a circuit board (Fritzing REALLY helps) or get some perf board with a million jumpers, but it WAY cheaper and not that hard. It does take some elbow grease though and design though. That's kind of what you pay for with those other motor controllers though; you're paying the guy who thought up and designed those break out boards, then you are paying the manufacturing costs. If you're doing a project on an arduino you probably aren't suppe worried about profession quality, time, or convenience, so why not make you're own board? Plus is super rewarding to build your stuff more from scratch than to just kind of buy things and stick them together.