Boxing matches are supposed to be held in Kokugikan Arenae.
You can go there by subways, Yurakucho Line and Toei Oedo Line.
Get on a subway traion at Toyosu station, “豊洲” then get off at Tsukishima station, “月島” which is the next station. Then change trains there. Once go out Yurakucho Line ticket gate and go through Toei Oedo Line ticket gate then get on a train which goes toward Ryogoku station, “両国”. Then get off at Ryogoku station. The train stops 3 stations before arriving at Ryogoku station. Buy one ticket for Ryogoku station at Toyosu station. It takes around 20 minutes and costs 280 Yen.
The arena is to the west of the station.
It takes about 5 minutes on foot from the station.