The books are currently not in print but we were able to borrow a pile of them through our local library. If you cannot find copies at your local library, editions of some the stories are available here at Book Depository.
Hopefully, however, the new movie of Ernest and Celestine will spark a revival of these sweet books, and perhaps more will be reprinted.
I watched this new movie a few weeks ago with my sons, and was not at all surprised to discover that the movie adhered neither stylistically nor thematically to the original books (except, of course, for the fact that both feature a childlike mouse & fatherly bear as main characters). Still, the movie had a charm all it's own.
I was especially intrigued by the story element in the movie which involves mice who retrieve children's baby teeth which have fallen out. I wondered whether this was a French tradition (similar to the tradition in other countries involving the tooth fairy) and so I did some research.
And if you are craving yet more bear & mouse stories after reading this post, I recommend getting ahold of some of these books by Bonnie Becker. So funny. Love them.