Anyone knows where I can buy tickets?
There’s an official tournament broadcasted on tv called "Oo Sumō"* six times a year (January, March, May, July, September and November).
Bu the way, in March in Ōsaka, in July in Aichi and in November in Fukuoka it's held a meeting. In addition, there are the ones held in the Kokugi Hall in Tōkyō.
Regarding the official tournament, the reservations for the next January are already open.
Tickets can be purchase through Nihon Sumō Kyōkai**, it's possible also to do an internet reservation or a phone one.
The meeting schedule is descripted in this site.
*Professional Sumō Wrestling
**Japan Sumō Association (JSA)