all the speed controller does is turn on and off very quickly to control the motor.

http://www.4qdtec.com/pwm-01.html

To put it simply, no, no it doesn't exist. Hook up the speed controller to your batteries. Change the travel adjust on the transmitter to 50 percent for the two channels you use for drive on the transmitter and the motors will only effectively see an average voltage of 12v.

Please note, that you should still respect the maximum limits that the sabretooth states overall.

This is the simplest method to run your motors on a lower voltage by tricking them into seeing the lower voltage. I have used this method several times myself and it doesn't require additional weight or parts in the robot.