Tannic acid can be made by simply cutting a handful of shavings or inner bark from the branch of a tannin-heavy tree, like red or black oaks, into a pot with water and boiling it. You'll know it's done when it turns brown from the tannins leaking out of the wood and it becomes an effective disinfectant. The only caveat is that you don't want to pour it directly onto a wound. However, you can use it on your hands to prevent an infection when working around the wound. It also works great as a bodywash, mouthwash, sanitizing your hands and tools before processing an animal, relieves inflamed or itchy skin, and there's many other uses. It'll stay effective for a few days at room temperature.
EDIT: As a note, this also works with other types of trees like birch, willow, or hemlock but you'll need to add more shavings in comparison to trees like oak to achieve the same concentration of tannins.