Johnny and Ponyboy located Dally at Dally's friend Buck's party.  Buck was Dally's friend and rodeo partner, and it was his car that Dally borrowed to go check on the boys in their hideout a few days after the boys escaped.  Buck, who was a bit older than all of the boys, even Darry, was not mentioned in the novel again.  It was an interesting revelation about Dally's character that he even had a rodeo partner--Ponyboy's first description of Dally early in the novel focused on his generally illegal recreational activities such as gang fighting, and the idea that Dally would be involved in a sport seemed a bit out of character.