What kind of light bulb does this lamp use?
IKEA provides a tool to help identify bulb size. The one you need is listed as a group: (E12, E14, E17 or SES)

The 12 in E12, for example, means a diameter of 12mm. This is just a bit under 1/2". Carefully measure the size of the threaded cylinder and you should know which size you need. E14 will be about 9/16". E17 will be about 11/16". E14 and SES are the same. 

E14/SES seems to be the most common and is probably your huckleberry.