Where can I buy books on cassette tape and cd online in the Italian
language?  I want to listen to my favorite American novels (like
Grisham) read in the Italian language , and to have access to
best-selling Italian books in Italian on tape or cd.

Not good news, I'm afraid. Apparently, audiobooks are hardly known in
Italy except as a service for people with poor sight, and there
doesn't seem to be any satisfactory answer to your question.

There are a few works of Italian literature on tape, but the format is
such a novelty that the website selling them explains - in Italian and
English - what an audiobook is and why someone might want one:
http://www.ilnarratore.com/ita/FaqAudiolibri.html
http://www.ilnarratore.com/eng/audiobooks_why.html

I also found a comment about how popular books on tape and CD are in
the US and how unusual in Italy:
"Questo a sottolineare l'enorme importanza di un settore, quello
dell'audiolibro, in Italia assolutamente marginale quanto ampio e
fiorente negli Stati Uniti, dove un libro č disponibile spesso in pių
formati."
http://www.alice.it/news/news/n20020110.htm


In Italy, people with bad eyesight can find audiobooks, supplied by
voluntary organizations, at libraries. In case you'd like a list of
charities providing audio books (audiolibri or libri parlati), here's
a webpage I found:
http://www.comune.venezia.it/letturagevolata/contesto/index.asp?scelta=centriparlati#bct

If you want to see what's been recorded by volunteers here's a catalog
search page:
http://www.comune.venezia.it/letturagevolata/catalogo/index.asp?cosa=parlati
(You can type in an author's name, e.g. Grisham, without needing a
title.)

Here's the site where you can buy a few audiobooks in Italian:
http://shop.ilnarratore.com/?locale=224
http://www.ilnarratore.com/index_eng.html

Hope this isn't too frustrating - 

Leli