The oils from cedar and pine are toxic to them, but you should be able to use any other tree. Hardwood is probably best such as oak or a nice manzanita branch. The branch should be dead and smooth without a rough bark surface that can get soiled and might be hard to clean.

To sterilize it you can place it in an oven set at 200 degrees for 20 minutes. That will kill any germs or bugs that might be on or living inside the branch.