Convenience stores and even ¥100 shops sell chargers. The ones at the ¥100 shop are about ¥770 for USB C (less for A), and the ones at the convenience stores are twice the price or more. Most stations also offer portable charger rentals, too.

When I first moved here, I brought a power strip from my home country and just used that with an adapter. It worked well but YMMV.

Hope you have a great trip anyway.

Buy a gallium arsenic charger, and just use it in Japan. I had no issues plugging in my charger in Japanese plugs. I even bought a anker gallium arsenic charger in Japan, and it had US style plugs.

From a security perspective don't plug any device you really care about into a USB plug you don't know. There could be malware on the other side of the plug, it's rare but it's real. Better to use your own charger, or if you're paranoid, you can buy USB charge only cables. But that's yet another cable to carry around

Hey! (1) Existing ones: If you already have the chargers with the Japanese plug shape (looks like [ I I ]) and support 100v/50 or 60 Hz input (most morden chargers support all the voltages over the world) then you can just use them.

(2) Convenience: If you want to buy one in Japan for cheap, just hop into the 100 JPY shops or convenient stores everywhere. They sell cheap ones which most probably cannot fast charge.

(3) Care about choices: If you care about fast charger, you can go to the Yodobashi Camera/Bic Camera/Yamada denki/LABO or whatever electronic goods stores. They will have plenty of choices

(4) Convenience and quality: you can go to the convenience stores and buy the Apple chargers (with Apple store price) and fast charge capabilities. Because you can always trust the quality rather than some unknown brands that the stores have with their limited stocks.