The power screwdriver suggestion has some merit, just don't go cordless. You can get a beefy corded power drill with enough torque to run a mill for real cheap. Just enclose it in a wooden box (with ventilation holes for heat) so customers can't see your hack job. Mounting is easy, just use some U-shaped bolts or metal straps. Bolting down the handle will keep the drill turning the mill and not vice versa. Tap into the potentiometer in the trigger that controls the speed and replace it with a casually hidden dial, so you can tune it to run at the right speed when grain is supplying a load. Shaft attachment is easy, just close the chuck on it. Future repairs are super simple; just get another drill when one breaks, tap your dial into the trigger again, and strap it down where the old one was. The only exposed moving parts would be a smooth spinning shaft. If a customer finds a way to hurt himself on that, he was probably flirting with death just operating the front door of your shop. (vs. an exposed belt which can remove fingers.)
I have a power drill that would probably drive two grain mills, way higher end than what you need, and looks like that model costs
about $100 from Home Depot. I see another one that would probably work fine for $60, and even cheaper ones from other brands. If you're building this, you probably already have a power drill you can test to see if there's a speed setting on the trigger that drives the mill well. It's worth a shot.
I've used my DeWalt to drill a 1/2" hole through hardened steel in the base of a full-height server rack. (Steel that holds up 4,000 pounds of equipment.) If I wasn't quick to release the trigger when it catches, it could easily wrench itself from my hands. I don't have a way to measure torque, but would guess at least 200 inch-pounds, possibly 300 given I'm holding it by a several-inch handle and it can still break free. The max RPM is in the 300s, but that's full bore with no load, and it went through that server rack at speeds closer to 80 to 100 rpm.
There's also this place. I saw a few motors getting close to your needs. They also sell the raw motors used in cordless drills, and possibly compatible gearboxes too.
