There’s a dude on the northern part of flat irons lake you can talk to that gives you a map with all legendary fish locations.
Close by the trainstation to the north west of that same lake you can encounter a guy in a blue coat standing next to a horse who sells you a treasure map for 10$. I strongly suggest you buy it as the reward if you take the time to complete it, is 1000$++. It will take you at least an hour to complete, even if you follow a guide on youtube, but it’s the fastest way of making money. There’s also a guy who finds a gold nugget where the treasure is, if you want, kill him and take it for an even bigger payoff.
Do the noblest of men and a woman quest. Gives you some cool looking weapons and also when you go to see Flaco, there’s a chest by his bed that starts another treasure hunt. Pays off 2000$ if I remember correctly but takes a loooooot of time to complete and the final part is a pain in the ass.
You will have to unlock a "fence" shop to be able to sell the treasures though. Which I believe you do at some point during chapter 2.