12V DC Motor Attachments
Well, first of all, that depends very strongly on which DC motor you're using. There's tons of them, all with different output shaft types and diameter.

Secondly, the attachments you're looking for are unlikely to be found commercially, you'd likely need to manufacture them yourself. This is because of my next point:

A plain DC motor is a poor choice for this system (and, truthfully, many systems). You have effectively no feedback system (that you've mentioned), so you don't know in your code how far the motor has moved. You can estimate with some measurements, but it's usually very inaccurate. You're not going to be able to move the arm in any sort of reliable manner with a plain DC motor. You'll also have no way to "hold in place", as there's no holding torque from an unpowered DC motor.

You likely want to look into small hobby RC servos. They're easy to control with Arduinos, have tons of commercial atttachments, are easy to manufacture custom attachments for without metalwork, and cheap. If you need something higher-power, perhaps a stepper motor will work, but you'll likely have the same issues with finding attachments.