How to duplicate Weights to different Bones WITHOUT painting? Foreword: I picked up Blender like a week ago, tops, specifically for this model.

Summary: I want to take pre-existing weights from one half of a model, and mirror them to their counterparts on the other side of the model. Not painting new weights, which is the only thing I could find info on.

I'm working on fixing a model for XNALara because it's the only one of its kind and its rigging is 20 flavors of messed up on the right half. However, no amount of googling or experimenting with any and every "mirror" or "symmetry" related option I can find has helped me replicate the "mirror painting with .L and .R bones" effect with existing weights. 

This was something that was incredibly easy to do in 3ds Max when I had access to that in college, so the fact that I can't find any instruction on doing it in Blender baffles me slightly. What am I missing?
If you have the bones named as Leander suggests (i.e., a left and right side naming convention and all needed bones in place), you can delete one half of your model (the half with the weights you want to copy), and then add a mirror modifier. Move the bones on the mirrored side around to see if it's working correctly, then apply the modifier. The only time this won't work is if you have shape keys on the model, since you can't easily remove modifiers on a model with shape keys.