Lightly corrupt heroes can be good for this. I'm not talking villains with good publicity, but heroes who have a reputation for personal power and good deeds, but who are more in it for the glory than the work. They might allow low-end criminals or evils to operate under their noses for some personal profit, lord their power over the common folk they protect (not actually hurting anyone, but clearly throwing their weight around and such), and so on. They also might refuse to assist with serious threats, either because there isn't anything in it for them, or because they primarily only fight weaker enemies who they can easily beat and aren't willing to actually risk their lives fighting enemies that could actually have a chance of beating them. For a group of truly heroic individuals, these sorts can be pretty infuriating.

Also, consider people who could provide assistance in critical situations, but demand something in return for it even if the heroes winning is clearly in their best interests. Like, an army is approaching, the heroes go to the local ruler to get some more troops to fortify their next target, and the ruler will only do it if they do some favor like intimidating a political rival or whatever. The pettier the demand and the more haughtily dismissive they are of the actual problem the better.