In season 3, episode 4 of My Hero Academia, the song starts playing at around 20:22 when Deku starts up his 1,000,000% punch against Muscular.
I've been searching around but I can't find it anywhere.
What's the name of the song?
The song you're looking for is from the soundtrack of Boku no Hero Academia season 2 OST, by Yuuki Hayashi.
The name of the track is Koutekishu to Kaite 'Tomo' to Yomu. It is the 14th track on Disc 1 of the OST.