Travel Transformer from US to Japan (100V -> 120V) Voltage?

You almost certainly don't need a transformer, since 100V instead of 120V is almost certainly within the tolerance of the device, particularly for a simple heating device like a hot air brush.

Fun fact: even in the US, that "120V" can actually be anything between 104V and 126V at the "utilization point" where you plug in your device.

If you don't mind something heavy, you can also find transformers for this purpose on amazon.jp. They all look like typical made-in-China adapters of dubious approvals status. Also very heavy but maybe acceptable for a long stay. The advantage is that you could order it in-country if your gadget does not work properly.

If your Japanese is good enough to deal with their site, try searching for "????? ???". Some of the reviews are bad because they are trying to use devices with high surge currents such as compressors or refrigerators, which your device should not be.

You are most likely not going to find a suitable consumer-targeted transformer to go from 100 to 120 V with their respective tolerances. Most devices that people are likely to take with them on a trip from Japan to North America or vice-versa are well-suited to handle both nominal 100 V and nominal 120 V. While there is a (small) market for 100/120 V to 230/240 V transformers to go to or from the European standard from the Japanese/North American one the average consumer will not need any transformation from 100 V to 120 V.

On the other hand, Japan has a very big market for consumer electronic devices so if you have any doubts about your hot air brush not being able to handle slightly lower Japanese voltage I would suggest just buying a new one in Japan.

Japan also has a well-developed second-hand market so you may well be able to sell it off for a good price when you leave, assuming it’s still in good condition.