How do I get BibTeX entries from amazon.com?

Copy the ISBN to Ottobib http://www.ottobib.com/

Check out zotero.org. You'll find bib/bibtex discussions in the forums and useful information spread across the web (e.g., here).

Note that Amazon may not be the most reliable source of bibliographic details. At least, I recently had to submit corrections regarding the creators listed for one edition of Frankenstein...

Anyway, use care, and it's best to double check against the information actually on/in the item in question.