Do a google for MFA (model flight accesories) geared motors, they arenot that expensive and can be got with 1000's:1 gear ratio's, if your rotating disc has a drive belt around the outside of it to the motor shaft that would also give you a good reduction.
If you can sope with movement evey so often then you could use a couple of 555 timers to pulse the motor every so often.
If you need good rotational speed accuracy then google for sidereal drives, they are used on telescopes to track stars.