This is going to be a BIG collection. Feel free to add your own suggestions but please pick the right category and observe the conventions.

Electronic sources

Free eBooks and eStories

• Project Gutenberg - http://www.gutenberg.org
• Gutenberg Australian - http://gutenberg.net.au/plusfifty.html
• Blackmask - http://www.blackmask.com
• Online Pulps - http://pulpgen.com/pulp/downloads/index.html
• Gaslight - http://gaslight.mtroyal.ab.ca/gaslight

Commercial eBooks

• Blackmask Store - http://www.blackmask.com/catalo/default.php
• Fictionwise - http://www.fictionwise.com - Agatha Christie only so far...

Print-on-demand

• Blackmask Store - http://www.blackmask.com/catalo/default.php
• Lulu - http://www.lulu.com - currently has Freeman, Biggers, Milne, Orczy and Wallace

New books

Publishers

• Adventure House - [http://www.adventurehouse.com] publishes expensive hardback copies of otherwise hard-to-get material.
• The Battered Silicon Dispatch Box has books by several writers available in quality hardback editions - http://www.batteredbox.com
• Crippen and Landru - http://www.crippenlandru.com
• House of Stratus is no longer operating but their books are still available from distributors.
• International Polygonics is also out of business but their catalogue has been taken up by the Independent Publishers Group.
• Rue Morgue Press - http://www.ruemorguepress.com
• Ulverscroft Large Print books have reissued many classic detective stories, but their website is painfully difficult to navigate - http://www.ulverscroft.co.uk

Distributors

• Amazon - http://www.amazon.com
• Barnes and Noble - http://www.barnesandnoble.com
• Alibris - http://www.alibris.com
• ABEBooks - http://www.ABEbooks.com

Second-hand books

• BookMooch - http://www.bookmooch.com
• eBay - http://www.ebay.com
• Gaslight Books, Canberra ACT - http://www.gaslightbooks.com.au
• Alibris - http://www.alibris.com
• ABEBooks - http://www.ABEbooks.com
• www.DetectiveFiction.com - http://www.detective-fiction.com
• Google Directory listing - http://www.google.com/Top/Shopping/Publications/Books/Fiction/Mystery/
• Yahoo Directory Listing - http://au.dir.yahoo.com/Business_and_Economy/Shopping_and_Services/Books/Bookstores/Mystery/

Specific authors