Here's a suggestion:
Check out Jones on Stepper Motors, with links to manufacturers, controllers, distributors, etc. and a ton of information on how to control them. The control signals will come from your microcontroller. The control power will likely come from a Driver. If you use AVR or PIC, the manufacturer will have tutorials and white papers, with code, on motor control. Read about the different types of stepper motors, and how they're driven with a pulse train.
Then, pick your motor. Shop around on google to see what's available. Have an idea of what kind of stepper motor you want (bipolar and unipolar are popular). You are looking at what the holding torque is, and what the rated voltage and current are. I'm assuming your positioning can be quite coarse.
To decide what kind of holding torque you need:
Measure the amount of torque the motor has to have, either with a torque wrench or by estimation. To estimate: you probably the know the length of the arm that needs to move (say, 6" or 15cm), and the amount of weight that needs to be moved (say, 0.1lb or 45 grams).
- Multiply those numbers together to get 6" * 0.1lb = 0.6 ft*lb or 15 * 45 = 675 cm*g = 0.675 kg*cm)
- Multiply by some safety factor to give you some margin (anywhere from 1.3 to 3, 1.3 is probably fine for you, so 0.675 * 1.3 = 0.8775 kg*cm)
- Then you look for a stepper motor rated with a hold torque of at least that 0.88 kg*cm. That's its maximum torque if you feed its rated voltage and current. Now look at that motor's rated voltage and current, can you supply that? (As in, if it asks for 120VDC at 1mA, you probably don't want that motor.) If you don't like it, look for a different motor, or look more carefully at what you're trying to move.