Public TV’s lobbying unit, the Association of Public Television Stations, said today it expects to hire a new president by September. The Boston-based search firm of Issacson, Miller, which specializes in nonprofit leadership posts, will conduct a nationwide hunt. Jane Gruenebaum, a onetime congressional staffer who was executive director of the League of Women Voters and c.o.o. of the Center for Policy Alternatives, is heading the search, working with Gail Gregory. In March, John Lawson left APTS after leading it for seven years.