Please do your own research when you actually get them to make sure they will treat your needs.
These are just some products I usually have with me at all times.
In Japan Plan B contraceptives are only available with a prescription from a licensed doctor that handles emergency contraception as a part of their regular practice. (Japanese source)
If you are in a situation where you need this medication, follow the next steps.
Check the list of clinics offering support in your local area. This is a government resource but unfortunately is not available in English. Click on the prefecture you are in to find the list. I would recommend trying to translate the documents as needed.
https://www.mhlw.go.jp/stf/seisakunitsuite/bunya/0000186912_00002.html
Check if those clinics have English support OR find someone who can come with you to the appointment to translate
If possible, call to make an appointment. If you do not have access to a phone or if they do not take appointments, just go in person and ask if you can wait for an opening.