The harder part of this will be getting the money. The easiest and fastest way is to use treasure maps to find gold bars. You won't be able to do this until you begin chapter 2 and can roam freely. Each bar is worth $500 at a fence, which is unlocked after completing "The Spines of America" with Hosea.
I used the Poisonous Trail maps (4 bars) and the Jack Hall gang maps (2 bars). Each set has 3 maps you have to follow in order to find the treasure. I also used the strange statues puzzle for another 3 bars. That will give you a total of $4500. See the not-my videos below for how to find these.
Also, just southwest of your camp, on the east side of the river, is the burnt-out ghost town of Limpany. In the sheriff's office, behind the counter there is a lock box containing another gold bar. This should give you a total of 10 bars and your $5000.
If you also look for homesteads (look at the Breaking and Entering achievement), you can find some loot that will also help towards this.
Once you have $5000, save your game.
You need to find a place to spend it. I found the easiest place to spend all the money was the tailor in Saint Denis. Go through the catalogue, buying one of every item in every color they have. You should hopefully get to $5000 by the time you get to the trousers.
Here are some not-my videos to help you with the three treasure quests.
Best of luck!