This can largely be accomplished by equipping your heroes with abilities they wouldn't normally use, just in case. Part of constructing a good team involves making sure that your party can still function even when someone is out of position. If you can't scrounge up a main healer, a couple secondaries or a few sustaining heroes can do the job just fine.You may have had trouble in the past when your team is forced out of its regular position, like when your party is surprised. Once your heroes get more powerful in their tier group and you accumulate trinkets it becomes absolutely possible to get through a dungeon without a dedicated healer but its not something I would recommend for a newer player early on. Very strange, not always reliable, but undeniably powerful. If you have the Crimson Court, the Flagellant is a weird beast, somewhere between a primary and secondary healer although maybe also the best healer in the game and also the best at self-sustaining.

Leper, Abomination, and Houndmaster are the best of these, but Hellion also has a way to give herself a couple points here and there (its mainly used for other reasons.) Other heroes are self-sustaining and have a self-heal of some sort.

They can't sustain like a main healer since their heals are only for a few points each, but they can be used to support a main healer, keep an otherwise self-sustaining party alive, or take someone off Death's Door in an emergency. 'Secondary' healers include the Crusader, Plague Doctor, Arbalast, and Antiquarian. The Vestal has reliable, consistent healing, and specializes in healing the entire party at once, whereas the Occultist Wyrd Reconstruction is an unreliable but, on average, very powerful single target heal. The 'main' party healers are the Vestal and the Occultist.

Darkest dungeon ruins best party