Hey
Any kind of leaves, like @InvertsandOi! said, that you may find in a forest that look generally untouched. @InvertsandOi! mentioned you can stick them in the microwave which is a good idea. You can also bake any kind of substrates / sticks / barks etc in the oven for 10-15 mins on low heat (be careful when applying heat to anything potentially flammable) to get rid of anything that may be lingering on it. You can use a big old baking tray and load it up with the substrates, it works well.
Good luck!