My best take.
Haruka Amami - Boston Red Sox. She's the iconic character of Im@s, and Fenway Park is one of baseball's iconic stadiums.
Yayoi Takatsuki - ... Detroit Tigers, the Tampa Bay Rays, or the Chicago Cubs.
Ami/Mami Futami - Dead ringers for the Minnesota Twins.
Ritsuko Akidzuki - ... She's the Green Monster out in left field in Fenway Park. Or the Seattle Mariners if I had to choose. (But only because of their colors.)
Chihaya Kisaragi - Toronto Blue Jays. No brainer.
Azusa Miura - She's gonna be the Colorado Rockies, isn't she? Purple and black, thin air up there, and the stadium is named Coors Field. *sigh* Well, I don't have anything against 'em...
Makoto Kikuchi - Uh... Texas Rangers? Arizona Diamondbacks? I'm more partial to the Rangers.
Yukiho Hagiwara - Guh... Angels of Anaheim?
Iori Minase - Kansas City Royals
Miki Hoshii - ... The Oakland A's are probably the closest you'll ever get.
Hibiki Gahana - Pittsburgh Pirates
Takane Shijou - Houston Astros (but their colors are blue, orange, and white)
Ai Hidaka - Cincinnati Reds?
Eri Mizutani - Tampa Bay Rays?
Ryou Akidzuki - Seattle Mariners, or whatever Ritsuko is (unless she really is the Green Monster)
Yuki Himekawa and Yuuki Otokura - America's team, the Atlanta Braves
Sanae Katagiri - Milwaukee Brewers