Bounties, Rewards and Loot
A few of the ways your players can earn currency are: completing bounties that might be posted by the town guard for instance, collecting rewards for completing missions, quests or milestones in an ongoing adventure or loot collected from those they just put down. To the victors go the spoils!
Should one of your players have a character who is known to take and collect things from others, theft is also a possible means of gaining income; though at what cost? Characters are subject to the law of the land they inhabit, and most don’t look too kindly upon thieves… until they need a locked door opened.