First, 100 to 120 volts is a step up transformer. Minor point but thought I'd mention it.
Why is that a challenge? We spend thousands of dollars on power regenerators or even boutique power cords. Putting a cheap transformer in the power circuit is asking for noise and degradation of quality.
I haven't shopped for a quality transformers to see if a quality manufacturer makes one. But I did look for a quality 120 to 240 transfomer without success. PS Audio is the top brand here IMHO and I have contacted them about making one of their power regenerators that changes voltages and they seemed uninterested. It would not be difficult, they have the technology.
Jerry