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.
You can visit Kakaku.com which is a online shop price aggregator with almost anything you can think of.
Here is the link to e readers: https://s.kakaku.com/pc/ebook-reader/ Use any translation tool if needed.
Note that it lists prices from online stores which are usually much cheaper than what you can find in physical stores. Refer to the most expensive price listed.