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If it's a 16mm hex you should be able to find one quite easily at a well-stocked auto parts or hardware store. But, it will need to be used with your 1/2" drive socket wrench and perhaps an extension. Ideally, you should be using a torque wrench to properly tighten your car's lug nuts.
I ended going to Canadian Tire, Home Depot, Home Hardware, and then f-ing Partsource and couldn't find the right bit. Not a single one sold them. At PartSource I picked up a hexagon star pattern that fits snuggly into the lug nuts (like dcains posted but modified) so it gets the job done. Can't believe it was such a ***** to find!