What you really need is a DC motor, gear box and encoder. It's probably a tall order to find a combination of all three that exactly fulfill your specs so look for the separate parts.
DC motors typically run at speeds in the tune of 10-20k rpm, a shaft encoder would work great for you (as described above - one side of the shaft white, the other side dark, and optical detector tells you how many turns the motor shaft made). You'll have to get a 1:5,000-1:10,000 gear ratio to go down to your 2 rpm.