How to get paperwork for horses?
You cannot acquire paperwork for horses. You can, however, find merchants willing to buy a horse with no papers (at a less reduced price).
During chapter 3, you will be introduced to a horse fence. This is similar to the fence you unlock during chapter 2, except that these guys (only) buy horses.
For horses with papers, stables will buy them for more than a horse fence will.
For horses without papers, horse fences will buy them for more than a stable will.