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 GX24q-1 socket.
From Amazon
Osram DULUX T/E Plus, 13 Watt - 13W / GX24q-1/830
LED Bulbs
Osram 13W Dulux CFL TE 4 PIN Warm White - OS446981
Big on Electricals
Osram 13W Dulux CFL TE 4 PIN Warm White - OS446981