come along在此为短语动词，意为： to happen or appear as someone or something that might be used, chosen, etc.

She says she won't just marry the first man that comes along.

本句为倒装句，为了突出主语，主语后移至句末信息中心位置，along前移到come之前，让出信息中心位置。本句倒装是由时间副词then引起的。