Since ifixit dont seem to stock the inverter as s spare part I thought I'd mention that that well known auction site has inverters listed as low as US$20 so it may be an option for you to try for not much money
The LCD tear down guide on this site states
"During reassembly, the order of the inverter cables is interchangeable within each socket."
It may be possible to determine if it is the inverter or the CCFL's by swapping the upper and lower connectors... if the cables are long enough
WARNING - the CCFL operating voltage can easily exceed 1000V. Personal injury or damage to the iMac is possible if contact is made with this voltage
If the dark part of the screen moves it is the inverter and if it does not then it's the CCFL in the LCD.
It looks as though the LCD is the same/similar as used in the Dell SP2009Wc. If so The CCFL are edge mounted and there are 2ea top/bottom of the LCD. Looking at the cct diagram for the inverter suggests that the CCFL should run separately so 2 CCFL across the top not working suggests the inverter not the CCFL as I'd say it was unlikely 2 CCFL would fail at exactly same time... This suggests inverter to me...
My guides - Removing CCFL Assemblies from iMac Intel 20" EMC 2133 and 2210 LCD (part no LM201WE3) and Removing CCFL from iMac 20in LCD CCFL assembly were inspired by this question
Is it the top 1/3 or top 1/2 of the screen? Does it fade from the middle and get darker as it goes to the top?