Your back yard is usually a good place to start looking. As long as no chemicals have been sprayed on them, they are fine to use. Give a good wash with some dish soap, blast with the hose and let dry in the sun.
You don't even have to clean outside branches, but if it helps you sleep. I've had about 20 different bioactive enclosures with branches that were no more than rinsed with the hose. Never had issues and don't know anyone that has. Most issues actually come with sterilized branches. There is no natural and harmless fauna/flora to keep fungus in check. The boring beetles also like dead wood.
Here's a little experiment for you guys. Go boil a piece of cork or whatever, place it in a high humidity bin. Now do it with a piece of wood that has been buried in soil or outside for a decent amount of time. Tell me which gets covered in mold/fungus.
get them from outside, don't waste your money on over priced store bought, cause guess what they were sourced from outdoors!!! Take a walk if you have some woods nearby and get to choosing!!!!! I have intergrated a big one, which she loves for basking as she can just chill and lay out under her basking light and conserve energy as she doesn't have to balance like on the smaller sized branches. Then of course I have branches that are just the right size for her little paddy, claws to comfortably grab onto. That is the important things to consider, not getting so called sanitized branches from a store. I wash mine with 7th generation dish soap, cause no harmful chemicals not that i worry, but we don't ingest them so why my animals. I keep my home eco safe without chemicals, so important for both us human animals and nature as well. So, with using that I know it rinses well and easily so no worries about any residues, also washed all my live plants with this as well and once again better for the living plant, and rinses off easily with no residue. DO NOT use Dawn dish liquid, anyone, even for your dishes, it has one of the same chemicals as RoundUp does, that is responsible for cancer. Thus why when you mix dawn with another ingredient, which i forget, to kill weeds outside, it is that chemical contained in the Dawn that makes it an effective weed killer. So just using my logic, i cut that product out of my life!!! We are all healthier for it!!!
I'm no expert, but from experience so far, and learning from all on here, just my 2 cents.