At the match game of Base Ball played at Litchfield, August 30, 1866, between the "Prairie" of Litchfield and the "Active" of Alton, the following was the score...(Prairie 64, Active 36)...-Alton Telegraph, September 7, 1866
The "Prairie" Club victorious by 28 runs. Time of game three hours and forty-five minutes. Umpire, E.L. Metcalf. Scorer E.T. Atwood.
Baldwin field captain of "Active" club, Forrester field captain "Prairie" club.
Playing for the Prairie Club was Forrester, p; Finch, 3b; McWilliams, c; Tuttle, lf; McAllister, rf; Zink, cf; Ayers, 2b; White, ss; Hood, 1b. Playing for the Active Club was Baldwin, c; Perrin, 1b; Sioman, cf; Morrison, 2b; Kellenberger, 3b; Dobelbower, rf; Clement, p; Smith, ss; Schweppe, lf.
Without checking my notes I'd say that this is the earliest reference to a baseball club on the Illinois side of the river that I've seen. It's certainly the earliest contemporary source that I've seen mentioning an East side club.