You can also order one from your dealer (cheaper than any online site for OEM parts)
I would suggest buying an aftermarket replacement from JT Sprockets or Vortex. It will be a lot cheaper and better quality than an OEM replacement. Just search "2012 ninja 250 front sprocket 14 tooth" or a combination of those keywords on Ebay or Amazon.
A 14 tooth sprocket is the original size, but I replaced mine with a 15 tooth front sprocket which lowered my RPM's and made for a much more comfortable ride