Heavy duty voltage resistor to decrease voltage from 120V to 100V
(1) So called "Auto transformers" which are essentially a single winding with a tap part way down the winding, are available which "buck" or boost the input voltage by a small amount. These are substantially less costly than a normal transformer of the same input power as they are effectively transforming only the voltage difference.  

(2) A resistor can be wound with suitably heavy Nichrome wire. This is usually available for  electrical suppliers. A self supporting air cooled coil or coils is easy enough to make. 

If you are converting from 120 VAC to 100 VAC at 100A the dissipation required is about 2 kW. That is 100 to 200% of a typical toaster, giving you some idea of what can be achieved. It may be possible to use toaster elements with rearranged wiring to parallel several sections to lower resistance. 

(3) Incandescent light bulbs may be useful. Their wide cold to hot resistance change may make their use hard.

(4) A series inductor may work depending on the load characteristics. An old microwave over transformer (or several) with added airgap to lower inductance and increase current handling. These often have welded core laminations. An angle grinder + hacksaw may be needed.