You're in luck, you can use just about any kind of servo!
Note that many of the photos below don't show the additional motor horns, but every servo comes with plastic clip-on parts!
People often ask us what they can do if the wire to their Servo is to short for their project. Not a problem! These cables act as extension cords - now you've got plenty of room.
The most popular/common servos have plastic gears, they're plenty strong and not too expensive!
These can go back and forth, rotating about 180 degrees
They come in 'standard' size:
And 'micro' size, not as strong but much more compact
These servos look a lot like the above but they rotate all the way around. Unlike standard servos you can't control the location of the horn, just the speed and direction it which it turns. Good as an alternative to DC motors for wheeled bots. For that reason, they tend to get purchased with matching wheels!
If you need more power, metal-gear servos can give you better torque, but at additional cost (since the gears have to be machined)
These are not continuous rotation