I shopped around, couldn't find anything in the local home centers. However, I did find some reflective aluminum.
I also looked at AH Supply's reflectors, but with shipping to Japan it would have been expensive. I was trying to keep cost under 15,000 yen. Anymore than that and I would have just bought an ADA solar II or something.
But the white paint is pretty brilliant and it was cheaper than buying several rolls of the aluminium stuff I mentioned earlier...