Charging a small 100uF 450V capacitor with a small battery?
What you could use is a boost converter such as the LT3573: -



This will charge up a capacitor to 300V. I reckon the parts would cost less than $20 and there are plenty of other similar chips of this type from Linear technology and TI for instance.

The circuit above produces 300Vdc and uses primary voltage sensing to determine the secondary voltage. Using what is known as a cockroft walton diode-capacitor voltage multiplier could get you much more than this quite easily. You would typically use this type of circuit directly on the transfomer secondary.

TAKE CARE - THESE VOLTAGES ARE QUITE LETHAL AND SCARY