They had several different sizes, including a couple of 24-volt motors. Prices ranged between $5 and $10 for the smaller ones -- sizes like you describe and a bit larger. In general, the larger the diameter you can use the more volts per rev you'll probably get because there's more room for magnets and peripheral velocity is greater for given RPM. I didn't see any 90-volt PM motors this time.
If you could find a power seat motor at a junkyard, that might work very well. I have one here that's 2.56" dia and would be a little under 4" long if one cut the wormgear drive off or made a new endcap. It turns about 113 RPM thru 44:1 worm reduction, which makes me think it would generate pretty close to 12 volts at about 5000 RPM. The armature resistance is only 0.175 ohms so IR drop would be very low. I think there's be no problem getting several amps out of it. It draws about 15 amps before it starts slowing down noticably.
AxMan had some windshieldwiper motors with gearbox attached, but they wanted way too much for them -- $19.95, I think, and you don't need gear reduction.