In addition to the companies Jon mentioned above, Ironclad Miniatures, Victorious Miniatures, Castaway Arts Miniatures, Redoubt Miniatures, Old Glory Miniatures, Tiger Miniatures and Steve Barber Miniatures also manufacture 28mm Boxer lines.
As I do not (yet!) have the Wargames Atlantic Boxers I can't do more than guess about compatibility. I assume since all the Wargames Atlantic figures are 3D designed that they would be very size consistent across their lines and since their Irish and Riflemen seem to mix well with Gripping Beast and Perry, the "middle 28mm" seems to be right for them.
Trust my WAG (Wild Assed Guess) or not, YMMV.