This SMPS boosts low voltage (5-20 volts) to the high voltage needed to drive nixie tubes (170-200 volts). Be warned: even though this small circuit can be operated on batteries/low voltage wall-worts, the output is more than enough to kill you!
EagleCAD CCT & PCB files
MikroBasic Firmware Source
High Voltage Switch Mode Power Supply (SMPS)/Boost Converter for Nixie Tubes - [Link]