I should have been a bit more specific: The Metric part refers to the bolt pattern for attachment to the hubs. In the States, there is still a huge portion of the sport, fairly collected in the circle-track segment, which uses inch fractions measurements. The wheels for each (Euro = Metric, American = Inch) are sport segment specific, although crossover is always possible.
So, what I want to know is: Where can I go most easily (website with online ordering?) to obtain 6″ Metric bolt-pattern kart racing wheels?