Well, from some of the replies, I'm baffled because if you go to Adafruit you can get a continuos rotation servo.
These are standard servos which have been modified in some manner to allow continuous rotation; using the term "servo" with these is kind of a misnomer, as they are no longer servomechanisms, but rather gear motors with a built in controller for direction of rotation dependent on a specific PWM signal for command input.
It used to be that you had to modify a servo to get it to continuously rotate, but now some manufacturers sell them pre-modified (they can be useful in certain R/C contexts), or vendors modify them and then re-sell them as modified.
I have never ordered from them, but they seem to offer very cheap servos (not as cheap as direct-from-china ebay sellers, but almost as good), plus a lot of other hobby R/C parts:
These guys are expensive, but they offer some interesting higher-power servo solutions: