If the official Assimil audio is unavailable, learners often use these free alternatives that utilize the same "Natural Method" or "With Ease" philosophy: Language Transfer : A highly-rated free audio course available as an app or MP3 files that mirrors the intuitive progression of Assimil. Ayan Academy : Offers free audio recordings for the first 20 chapters of L'italiano Secondo Il Metodo Natura
The most famous theoretical backing for the Assimil style is Stephen Krashen’s Input Hypothesis
If you prefer "good paper" (physical books), the modern editions of Italian with Ease
ascolta il dialogo senza testo per familiarizzare.
For those interested in the classic "Without Toil" versions of the course, historical materials are sometimes available through educational and archival sites. Internet Archive : Digitized versions of older editions, such as Assimil Italian with Ease , can sometimes be borrowed or streamed via the Internet Archive University Repositories