I purchased a replacement I/O assembly for my wife's Nexus 7 a few weeks ago on Ebay. It was from [http://www.ebay.com/usr/foxpcparts|foxpcparts]. I liked them because they shipped from Michigan (I'm in the USA). They aren't selling any today, but there are a few others that ship from China and Hong Kong. Just search for:
"nexus 7" usb charger port
By the way, it works great. The nexus charges much faster and the charger stays plugged in more reliably (not perfectly, but definitely better).