I don't know how he found them, but Darcy deffinitely went looking for Elizabeth and the Gardiners. He wanted to show Elizabeth that he could be polite and had taken her comments about his rudeness to heart. He went out of his way to show civility to Elizabeth's relatives. I don't think he did so to win her at that point, I don't think he thought he stood a chance of winning her at that point. But he knew that her comments about his impolitic behavior were justified. He wanted to show Elizabeth that he had improved.