Another thing, you can bake the moisture out of the flyback. This has a good chance on curing the problem, as I have done this in the past with older TVs. The problem is dust and dirt attracted by the high voltage, and it carries with it, moisture... It gets into the focus/G2/Tripler block which is integrated into the flyback, and you get the symptoms you see...

As far as a replacement, its obvious that you wont get the exact one. However flybacks are still very available for other TVs, and etc like sony monitors. and especially RCA TVs.

The trick here is, on a working imac, the secondary voltages/pinouts of the original flyback needs recorded, as the secondary voltages are used for the scan drive circuitry, and x-ray protection. Once the pinouts are figured out, and the voltages are figured out, You can match one up. Because most of the TVs that flybacks are still available for, service information is out there and you can look at the schematics/service data to figure out if those flybacks will mostly match up. Then once you find one, its only a matter of making a "mate" PCB to change over any mis-pinouts. As long as secondary voltage windings are close enough.

This kinda falls in james department ;-)