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

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

New books

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

Berrow, Norman	Ramble House
Boucher, Anthony	Ramble House
Derleth, August	Battered Silicon Dispatch Box
Fowler, Sydney	Sydney Fowler Wright
Freeman, R Austin	Battered Silicon Dispatch Box
Halliday, Brett as Dennis Lynds	PointBlank
Heyer, Georgette	Random House UK
Holding, Elizabeth Sanxay	Persephone Books
Keeler, Harry Stephen	Ramble House
Keene, Day	PointBlank
Ozaki, Milton K	Ramble House
Popkin, Zelda	Boson Books
Prather, Richard S	Fictionwise
Rogers, Joel Townsley	Ramble House
Seeley, Mabel	Afton Press
Starrett, Vincent	Battered Silicon Dispatch Box
Talbot, Hake	Ramble House
Taylor, Phoebe Atwood	Countryman Press
Whittington, Harry	NuovoBookstore