What is the best way to get BibTeX entries from ISBN number?

Did you try Lead2Amazon?

Select BibTeX, enter your ISBN, and click on "set and search" -- done.

Sometimes you need to try other Amazon sites, e.g. when searching books of Herbert Voss ...

Search for ISBN and export to BibTeX

• Ottobib: http://www.ottobib.com/
• similar to Ottobib: http://manas.tungare.name/software/isbn-to-bibtex/

Search in all fields and export to BibTeX

• needs many mouse clicks until you can export: http://www.citeulike.org/

Famous but needs repair

• lead.to/amazon select Amazon.co.jp and Bibtex. This combination still works today.

All databases have many mistakes in the fields and you will have to double check always. But don't worry the major science journals provide bad BibTeX data too.