Copied from http://www.ehow.com/how_12169328_track- ... rvice.html
1. Look up the serial number for your Nintendo DSi. If you didn't write it down when you acquired the device, check the purchase receipt and the box in which it was packed. If you no longer have these items, contact the store where you purchased the device, as it is possible that the serial number was recorded at the time of purchase.
2. Open your Web browser and visit online auction sites such as eBay, uBid.com and Overstock.com. Use the search tool on each site and type “Nintendo DSi” into the input field. Sort the results chronologically and review the most recent postings. Examine the pictures for each product listed and try to discern the serial number. If you find a Nintendo DSi with your serial number, contact local law enforcement.
3. Review online ads posted on major classified websites such as Craigslist. Search for recent Nintendo DSi listings. Look at the pictures and try to read the serial numbers. If pictures aren’t available, email the advertiser and request pictures. If you find a Nintendo DSi with your serial number, contact your local authorities to recover your stolen device.
If this doesn't work, I guess contacting Nintendo is the only other option.