When you Activate the Ancient Wardstone at Broken Toe you will notice a broadcast message that says "Defiant: Bahlmus of Shimmersand has arrived at Broken Toe and appears to be in need of assistance." Go to the bottom of the switchback to meet Bahlmus and acquire this quest as well as Spared Parts (29).
You will have to get to the top without losing Bahlmus or Claptrap while being assaulted by wave after unending wave of Gnar! When you get to the top he will sell to you!