Does anyone know where I can find a 100 volt input/100 volt output constant voltage transformer, somewhere in the 500 va range?

 

Try a switchmode power supply.

 

Yah,
Look for a Sola distributor. I've used these transformers on control panels before. Keep in mind they generate lots of heat from operating in saturation.
Malachy

 

Don't know your location but in the USA about any Electrical Supply Co. should carry what you need. Some manufacturers are Acme, Dongan, GE, SqD, Staco.

However, your specified input/output voltage, is not typical of an off-the-shelf unit. Usually the input is stated as a range, eg, 95-135 Volts, while the output is fixed, eg, 120 Volts.

Regards,
Phil Corso, PE
(Boca Raton, FL)