Adjustable Battery Power Supply for Cheyenne Hawk Pen Tattoo Machine (6-10V, 2A)

I have a tattoo machine that runs around 8-9 volts and the power supply outputs at 2A. What type of dial can I get that would allow me to adjust the battery output from the full 9-10 volts to 6-7 as sometimes I have to soften the machines operation to create smoother fades in the artwork. Also I would like to have a on off button and I am not sure if I would need any further controls to regulate the current or one dial setup would be sufficient... It's to attach to a Cheyenne hawk pen, if you need reference there's lots with a quick Google search... Any help or suggestions on batteries as well would be much appreciated I can head to a local electrical supply chain here and assemble whatever's necessary, just the guys at the counter always tell me they don't have time to design my project for me... Any help would be greatly appreciated 

Hi,

I was thinking you could do something similar to this? http://www.instructables.com/id/Mini-adjustable-power-supply/?ALLSTEPS

Yes that is basically what I need with a battery connected to it, so I can run my cheyenne hawk pen wirelessly... Is there a way to simply put a potentiometer for the voltage and have the amperage remain constant? I am trying to make a wireless power supply for the tattoo machine and will build several so they can remain charged and ready to go... I really just want to make a battery with the right potentiometer that allows me to turn it on and the up all in one device... The guys at the local electric shop wernt very open to discussing details of the potentiometers and there were so many....