There is always the material in the Oort cloud and Kuiper belt that could be used. Likely not enough to build a Ringworld, but it is something. Also, what is wrong with assuming that solar energy is converted into matter? Replicators anyone? Heck, Crystalline races have facilities that do this explicitly already.
The problem with using solar energy to make matter is it takes an absolutely ridiclous amount of energy to make a small amount of matter- you're basiclly undoing the matter/antimatter reaction. E = mc^2 is nice for bombs but a pain in the butt when trying to make matter:
m = E/C^2
From a game perspective, you can mine the ring/sphereworlds because if you couldn't they'd be pretty much useless.
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Phoenix-D
I am not senile. I just talk to myself because the rest of you don't provide adequate conversation.
-Digger