Rollerderby
There's countless ways that Sora could have encountered this obscure sport. The famous movie featuring it, her friend currently living in the states mentioning it, individual investigation of a new sport after tennis, maybe during one of her and Taichi's many adventures...
Sora was unlucky enough to find out by being bowled by a random foreigner in skates.
The girl obviously hadn't known much Japanese but was sloppily apologizing with a terribly thick accent while being lectured by a nearby old man. Apparently the old man had told her a few times before that it was dangerous to be skating around.
The foreigner didn't really seem to understand a word of the lecture.
Sora was by no stretch of the imagination fluent in English but she had taken her classes seriously and could manage simple, short conversations. Her pronunciation was terrible though.
The foreigner brightened at words she could actually understand, explaining sloppily in both broken English and Japanese that she skated around for 'practice'.
Sora found herself curious. Somehow the foreigner seemed to catch on to this and before Sora really knew or understood what was going on, she'd been invited to watch a Roller Derby match.
It was love at first sight.
Most of the teams in Japan are primarily English speaking and located on military bases but Sora managed to find one that had an Japanese coach and practiced at Tokyo Dome.
This is how Sora became a Tokyo Bomber Girl.
(additional and unrelated info on Roller Derby below the cut)