Not quite sure what you mean by classics since people use that word in many senses, but I like to find them in used book stores where the prices are lower. However, if you are searching for a particular title , it would be quicker to use AbeBooks or Alibris and find it in a UK bookstore online.
I always buy Wordsworth editions on Amazon
Try amazon?
http://abebooks.com/
Not quite sure what you mean by classics since people use that word in many senses, but I like to find them in used book stores where the prices are lower. However, if you are searching for a particular title , it would be quicker to use AbeBooks or Alibris and find it in a UK bookstore online.
Amazon
Library, bookstore, Amazon
Waterstones also has a section with beautiful covers!
Bookstore or Amazon usually
http://bookdepository.com/
You can find anything in eBay.