If you really want to put in a rheostat, you should figure out what the voltage vs. current relationship looks like for your motor. Since you have a variable power supply, take some points from 0-28V using a voltmeter (across the motor) and ammeter. From those points, you can determine the maximum power that would be dissipated by the rheostat.
One point, for example:
If Vmotor = 16V and I = 4A
In this condition, the rheostat would see 28V - 16V = 12V. So it's power dissipation would be 12V * 4A = 48 Watts. The resistance would be 12V/4A = 3 ohms.
Do that for about 10 points and see what the max dissipation and min/max resistances would be. I'd look at Digikey, Newark, Arrow, etc for suppliers, unless you have an electronics store in your city... sometimes they sell parts for appliances, and have shelves of general electrical components, too (like Radio Shack, but with more selection).