Energy-efficient light bulbs use, as the name suggests, less energy than certain other types of bulbs. The socket on this bulb is a so-called 2G11 socket.
From Amazon
Fluorescent lamp 2G11 Neutral White Dulux L55W/840 Original, no plag
Big on Electricals
Osram Dulux 55W PLL 2G11 Cool White - 4050300295879