Their numbers are legion these days. Apparently modern science
has improved the old "hobby motor" to the extent that you can buy a
very small, very powerful 12-volt motor off the shelf at Radio Shack
(or, more cheaply and with greater variety, from the catalog of All
Electronics Corp, http://www.allcorp.com, highly recommended).
The trick to any of this, however, is the speed control: you'll either
need a solid-state pulse-width controller, which you might or might not
have to build, or a big, expensive, hot, and generally-unattractive
rheostat, or some other more clever option.
My own solution would be to use a group of incandescent lamps; 12-volt
taillight bulbs would be good. Place them in series with the motor and
your voltage source and short out a lamp whenever you want your roll to
move a bit faster. Haven't tried this for this application, but it's a
generally good solution for any high-current motor speed application
where precision isn't critical.