Rotting wood chunks are great in your soil mix. You're looking for spongy, wet, crumbly stuff you can break apart with your hands. Sometimes when I gather this stuff up I find a colony of ants, or some gross looking grubs or centipedes tunneling through the wood. If this happens, I don't gather up that wood! I move on looking for some bug free stuff.
Leaf mold that's super broken down, from around the base of big trees or old stumps is really great too.
If you're still concerned with bugs, leave your forest stuff outside, in a bucket for a week or two so any critters can crawl away. Or lay out a tarp and spread the forest litter out in a thin layer so bugs can't hide in there. Then give them a couple days to flee the scene. If that's still not enough, sprinkle some neem meal on top to drive bugs away.
You can mix these things in soil, use them for mulch or top dress with them like compost. If you're into aerated teas some leaf mold from the forest and some worm castings will make Double Dank stuff.
You don't necessarily need to go deep into a nature preserve to find this stuff. Any shady area with trees and wood on the ground will work. If you can find a nice sloping area facing away from sunshine it's sure to be moist and good for fungus. You could probably find some body with a fireplace or wood-burning stove and dig around for rotting wood under their wood pile.