Shunted sockets are fluorescent sockets with an already install shunt connecting both pin connectors. Old style fluorescent sockets and ballasts use(d) non-shunted sockets.
Here are a couple of sources
http://www.horticulturesource.com/su...-shunted-p410/
http://sunlightsupply.com/p-12157-t5...t-sockets.aspx
http://www.aquabuys.com/p/ss_t5_socket.html
http://www.reefshops.com/waterproof-...eef.html?sl=EN