Short Answer: The boys look for Dally Winston immediately after Johnny kills Bob Sheldon.

In chapter four, a group of Socs attempt to drown Ponyboy in the fountain of a local park, and Johnny saves his life by stabbing and killing one of the Socs holding Pony's head underwater. Immediately after Ponyboy regains consciousness, he begins to panic and frantically asks Johnny what they are going to do. Johnny mentions to Ponyboy that they need money, a gun, and a plan. Johnny then comments that they will be able to attain all three things if they find Dally. Johnny has faith that Dally will help them out and come up with a plan for them to skip town before they are arrested. Johnny and Pony then travel to Buck Merril's place, where Dally gives them a gun, fifty dollars, and valuable advice. He instructs the boys to hop on a train headed towards Windrixville and hide out in an abandoned church on Jay Mountain until things settle down.

After Johnny and Pony accidentally kill Bob Sheldon, they are completely freaked out and they think they need to run away so they do not get arrested.  What they do then is they go and look for Dally.  So the answer to your question is Dallas Winston.

They figure that Dally is the member of the gang who has been in the most trouble and he would be the one who is most likely to be able to help them escape.  Johnny is the first to think of it -- he says Dally can help them.  Pony agrees, thinking that Dally can do anything.