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hellolighting or marinedepot carries sheets of highly reflective material used for aquarium lighting. However I don't think that type of refrector will be good for MH you need something parabolic so the light is focused down into the tank.
I would get the ones that are already pre-bent to shape. I use 2 spider reflectors that work well. I think they run about $30-$40 a piece or less. Getting that stuff bent to shape is a pain.