How can I generate and export the support structure for an object as a mesh? I want to render a visual of an mesh I created with a real support structure that a 3d printer would print. I haven't had any luck in finding a way to do this. Is there a program (ideally free) that can generate the proper support structure and export it into a mesh format, like .stl or .obj ?
You could try Meshmixer its free and the supports generated with it are embedded into the 3D model.

Here you can find a reference on Meshmixer supports. Meshmixer is well known for making custom supports for complex 3D models.