Might want to check where they sell supplies for the movie industry. If you ever watched the Star Wars DVDs, you'll notice they carry around a half-chromed sphere to get lighting references. It's possible you might find one there.
Otherwise, another way to do it would be to go buy a metal globe of the world then take it to your local chrome plating specialist.
Professor of Digital Arts
Digital Media Arts College
Boca Raton, Florida