In addition to the other sources suggested here, check with the coach at the high school for suggestions. For example, my son's on his high school's wrestling team. There are a number of wrestling youth leagues in the area that feed into the school. One team actually uses the school's facilities to practice in. In another case, one of the assistant coaches is the coach to one of the other youth wrestling teams.
You'll find that, for many sports, it's a pretty small world out there. Contact a coach at the school and ask about extramural sports teams and leagues.
Hope that helps.