Use energy-efficient light bulbs if you're looking to cut as much energy costs as possible. On this light bulb you'll find a GX24d-1 socket.
From Amazon
Osram 446929 Compact Fluorescent Lamp Dulux T 13W/830 LUMILUX Warm White
LED Bulbs
Osram Dulux 13W PL-T Plus 2PIN Warm White - 446929
Big on Electricals
Osram Dulux 13W PL-T Plus 2PIN Warm White - 446929