Just an idea...why not take a DA cog to a chrome plater? That's how they would have done the after market ones in japan. A plater will charge you the minimum because it's so small, which will probably be about $50. That makes for an expensive cog, but if you brought 3 or 4 of them, the cost for plating the lot would not go up much, if any. Then you've got some backups, or some to sell, or some to just hang out with and grab a slice at pizza hut.

But to answer your question, I don't know where to get a chrome DA cog or lock ring stateside.