You can wire a Trimmer/Potentiometer into any of the popular regulators used, including the one mentioned above and there are various styles available with knobs, the screw type and the wheel type.
Check out the Modding Forum/Batterty Mods Section for some info and ideas. A popular thread in there is the EverCool Variable Voltage
mod. That should get you going and answer many of your questions.
If your looking for an all in one solution with the regulator, board and potentiometer already completed check out the DE-SWADJ3. It's probably the easiest solution for a build but it's expensive. I can build a mod for that that regulator cost.