Some travel centers, such as the Japan Travel Centre (UK), offer cheap tickets for under 25s, so call your local travel agent specializing in Japan travel and ask for any deals.
Here are our recommendations:
Budget buses to/from most main cities, such as Tokyo.
Reservation Website (click link at the top for English)
JR Bus Kanto
The biggest bus company, often with the best prices. As the website is only in Japanese, it’s best to reserve at a main JR (national rail) station such as Tokyo station or ask you hostel/hotel to reserve it.
Reservation Website (Japanese only)
Buses all the way from Kyushu! The online reservation is only in Japanese, so ask you hostel/hotel to reserve it.