Robbing stagecoaches can be a profitable endeavor. Look for stagecoach routes and plan your ambush carefully. However, be aware of the lawmen who will come after you if you're caught. You can also loot the passengers and the cargo on the stagecoach for valuable items like jewelry and money pouches which can be sold for a good sum at the fence.
Doing story missions and side quests is also great. You get paid for completing these tasks. Some side quests can be really lucrative, especially the ones that involve helping out wealthy characters or recovering valuable items. Moreover, as you progress through the main story, you'll earn a decent amount of cash which can be used to buy supplies and upgrade your gear.
One of the best ways is to hunt and sell pelts. High - quality pelts can fetch a good price at the trapper or general stores. For example, a perfect bear pelt can give you a substantial amount of money.