I bought two sockets from
http://www.cheapcycleparts.com
They always have reasonable prices and fast service.
They were showing two 16 tooth sockets available, and I was not sure which one to get, so I bought one of each.
When they arrived, they are the same socket, just from different suppliers.
So, if you order from that site, just buy the less expensive one.
Your installation guide was awesome, and I did it myself in less than 40 minutes.