The cheapest reliable place I've found is here: http://www.parts4repair.com/asus-google-nexus-7-dock-charging-connector-flex-cable/ Not cheap but I haven't found anywhere that is better. They're new parts, I bought a couple from them and they warned me about an unavoidable delay in shipping, asked me if I'd prefer to wait or get a refund, and when they were arrived they were well packed and worked nicely.

I bought one of these to repair a 32Gb Nexus 7: http://www.parts4repair.com/asus-google-nexus-7-complete-screen-assembly/ Doesn't come with the bezel fitted so you have to use very thin/strong double sided tape to stick it to the old bezel.