Staring from 18 May 2019, the remaining ticket sales of Rugby World Cup Japan 2019™ is sold on a first come first served basis to all the public until July 2019.

All the popular fixtures have been almost sold out but some of the interesting fixtures are still available such as England vs Tonga at Sapporo Dome, Hokkaido where England beat Argentina in the 2002 World Cup football game.
If you missed to get a ticket for your-dying-to-watch matches, please note that the official ticket resale will start from 31 May 2019.

[From the official site]

This is to provide fans a safe and secure platform from which to resell Rugby World Cup 2019 tickets for matches they can no longer attend.

• The platform is not for trading tickets with other fans, or to sell tickets directly to any specific person(s).
• Resale on any unauthorized site is prohibited as being set out in the Ticketing Terms and Conditions.

Service Period Scheduled to start on Friday, 31 May 2019 from 18:00 (Japan Standard Time)

Visit the official website for further details.
Scroll down to 9) Official Resale Service
https://tickets.rugbyworldcup.com/showNews.html?changeLanguageTo=en&idNews=100