I don't know of any way to get rid of them without chemicals either. I do know that people think teatree is good but it is not good for dogs. I agree with Onyx that washing washing them (more soakng them without soap then washing) is going to help a little. There is a tablet available now that might be the least chemical option since it does not go on the coat.
If flees are bad you would have to treat the area your dogs live too and that would involve chemicals.
Any reason you don't want to use a product?