I posted a while back about using the main ferrite transformer out of a computer power supply. Just to report I've got one happily driving an 8 watt fluoro. One word of caution though. The core I'm using saturates very quickly at 20KHz; so I'm running it at 50KHz.
Second heads up. If you push the mark/space ratio too high, the tube goes into a forward conduction mode, and the circuit will draw heaps of current. To overcome this, put a resistor in series with the tube. I had on hand a 750 ohm 5 watt which is working fine, although a 1K 1/2 watt let the magic smoke escape very quickly.
Will post full details of what I'm doing, in my diary, in due course.
Amanda