The first thing Mattie has to do in Fort Smith is convince someone to accompany her into the Indian Territory. She chooses Rooster Cogburn, a federal marshal, to travel with her, and a Texas Ranger, LaBouef, comes as well. According to the book, Mattie said this: "Where can I find this Rooster?" She needed help and Rooster sounded like the best man for the job.
After choosing her partner, the next thing Mattie needs to do is get a horse, so she goes to Colonel Stonehill's barn and negotiates with him to get a horse.
Rooster and LaBouef try to keep Mattie from going into the Indian Territory by keeping her from using the ferry to cross the river next to Fort Smith. Mattie, however, will not let up, and crosses the river on her new horse.