Maybe you can find one in a used treadmill/exercise machine?
The old, direct-drive automotive starters are powerful and have been used in electric vehicles, but they are generally not rated for continuous duty. You might find something smaller (e.g., an old motorcycle starter), that can take the duty cycle you need.
DC motors from treadmills are probably rated for more duty. Although most today are probably rated for 90V or more, I bought one on a surplus site several years ago that was 12V. It was such a nice motor, that I bought lot lot more of them. Look around, you might find something a lot cheaper than the shipping cost.
John