When doing the quest "The Family" they are found inside the train station - go in and be careful for bear traps. When you find them don't kill any of them, find their leader.

When you have done that convince the kid to go back to his family.

Get the leader to ask the sheriff to make a deal for bloodpacks.

When that's done go back to the family's leader and talk to him, he will eventually give you the option to gain 20HP from bloodpacks.