Umm...for Adventurers the primary method of income tends to be, well, adventuring!
Here's some cash\loot-heavy options:
* Look-up the name of your local king, and send an offer to clear soem menace from his lands: goblins, orcs, etc...for a fee. Take their treasure, and get a reward.
* Try to find some ancient ruins inhabited by undead, then contact both a local church and a local wizard. Tell the wizard you are delving into the ruins and for a small fee, you will offer him first sale on any arcane artifacts (IE magic items). Then contact the church and make a similar offer about divine artifacts as well as a promise of vanquishing the unholy menaces below.
* Depending on the groups alignments, enslave a bard NPC and force him to perform in local bars and such until he has a reputation fo rgreatness, then force him to perform for kings and rich families...and keep the money for yourselves.
* Depending on the groups alignments, have one member kidnnap a local dignitary disguised (or polymorphed into) a goblin\kobold\orc, and then stealthily place him near the entrance to a goblin\kobold\orc cave (anywhere you know you can handle the creatures inside). Wait for him to be taken inside, and then once the famliy offers a reward for his return, go smash up the caves, take any treasure the creatures have, and return him for the reward.
* Depending on the groups alignment, use arcane\divine\psionic might to enslave the rulers of a small town mentally, and force them to legally turn the town over to your control. Set-up a tiny false democracy, and then have all members of the party elected to various high-level positions. Institute martial law enforced by mindless indead (or mentally dominated warriors who can't object) and turn the happy democratic city into a fascists city-state of slave labor..pick a good local products or two and force them to be mass produced, and retain 90% profit for yourselves.
See lots of options, and those are just in-game examples my group has used
