Then at their respective concerts, idols will throw autograph boards or autograph balls at fans at some time (that depends on luck).
Or go to places like Noboru takeshita. Many stores like jfc sell signature balls. I wonder if j-shop sells it. It's an official peripheral store (everyone's price is different. I remember seeing Renzhi's 10w yen before, and the others cost 5w. I don't know why).
Generally, you can't take photos and sign autographs when you meet them in the street because grandpa says it's to make money (damn grandpa = =)