What sensors/systems can I use to get position of body joints?

I'm interested in creating a Processing program that displays a stick figure whose joints (elbow, wrist, knee, feet, hip, neck), relative to one another, are based off of a human joints' positions.

I don't know what sensors to use to get this kind of information and would like some advice.

Image processing is not ideal here :).

Well, that's a very ambitious project!

If we exclude image processing, you need something like a human on a string, where you can tell the joint positions from the connected string lengths.

An exoskeleton may be a handier approach, where you can measure the angles between the joints. In this case you have to monitor both the linear (bending) angle as well as the rotational angle between any two joints, using e.g. absolute rotational encoders.

Image processing may not be ideal, but it is a lot easier than the alternatives. Many people attached IR LEDs to the joints and use an IR filter on the camera. You detect points of light in the images, and map them to the model (easier said than done).