This is hardly a comprehensive list but below is a list of
reputable sites for purchasing on-line:

Booksellers:

Multiple book price comparison - Try this first!

Austria (German and English)
Canada (French and English)
France (French and English)
Germany (German and English)
Japan (Japanese)
UK (English and US titles)
(English and Spanish)
(Amazon is not the same around the world)

Booksamillion (Often slightly cheaper and a good discount scheme)

CD's and DVD's:

CDNow

Tower Records

Sheet Music and Music Paraphernalia:

Di-Arrezo.com (Very comprehensive sheet music site with good discounts for teachers)

musicroom.com (Publisher specific: Novello etc but good)

Sheetmusicplus (Very comprehensive sheet music site)