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.