I’m looking around at motors and low cost options to get one since it seems this can be one of the pricier/harder to find options. So I’m curious, has anyone tried using a car starter?
I’m doing a little research, and it doesn’t look like there’s anything that says it can’t be done. Main concern is power pull, but with the tiny load I don’t think it will pull that much. I’m determined to make this work, car starters are all over the place in scrap yards.
I have not heard about using a car starter, but here is some inspiration for other cheap solutions for the motor. 
This guys uses a Garden Mulcher: Pretty cheap!
These guys are planning to use their hands: Super cheap!
Good luck and have a nice day sir!
in addition to what @andyn said, cooling would also be an issue, as the motors aren’t designed to run continuously. another option could be the motor from a garage door opener or similar
Starter motors are good for what they were designed for, starting engines. They deliver fairly high torque for a short time. Unfortunately they are not very efficient. It will draw 30-60 amps with no load and possibly over 500 at full power. It will get very hot if run for more than a minute or two continuously. And though they have high torque, it’s probably not enough to run the shredder directly, so you would still need some sort of gearing.
Most starters are also series wound-field types, so not reversible (swapping the battery connections won’t change the direction of rotation).
There are newer types that have permanent magnet motors (reversible) and some even have built in planetary reductions, these would be better, probably still not a good choice, but if you’re going to try this, look for one of these!