A Few Low Cost High Voltage Power Supply Circuits
Here's a circuit that generates 100V from 25V. Output current is 25mA. You can find it at http://www.edn.com/index.asp?layout=article&articleid=CA46343
Another high voltage power supply circuit is at http://www.instructables.com/id/Cheap-Semi-High-Voltage-Power-Supply/ It is actually just a high voltage multiplier circuit, and is open loop. In other words, you put in an ac signal, and you get the high voltage output.
This is a low cost 50kV power supply. Like the one above, it is open loop, however in this case it includes a way to vary the output voltage. It uses parts that are available at low cost. See http://www.instructables.com/id/Build-a-Variable-High-Voltage-Power-Supply/