Here is an example of one that will run on low voltage.
https://www.ebay.com/itm/Bodine-Gearmotor-32D5BEPM-W3/132733743604?epid=0&hash=item1ee78c31f4:g:Ev0AAOSwxHJbaMZe:rk:33:pf:0
This is an example of one you DONT want. The voltage rating is too high to step down at 14 rpm it would be about 1/2 rpm at low voltage.
https://www.ebay.com/itm/Bodine-Gearmotor-24D4BEPM-Z4-130-VDC-0-48-A-1-17-HP/132336856536?epid=1169968647&hash=item1ecfe42dd8:g:pYEAAOSw1KNZxD4G:rk:60:pf:0
The price on these has really jacked through the roof, this is all people trying to capitalize on makerbot stuff which wasnt around when I was using these. They did they same thing to cnc motors I used to buy on freakbay-someone figured out you could use them as windmill generators and the price went up 20x what they used to sell for. Crooks.