As well as the large well know general book suppliers such as Amazon, I've included here some more specialised military, etc. suppliers who may be able to source out of print military books.

Amazon - Possibly the best known on-line commercial book seller. This is the UK site - substitute "com" for the "co.uk" part for the US site.

Abebooks - An excellent book finder, a good place to start looking. As above this is the UK site.

Caliver Books - New and second hand books; includes Partizan Press and Matchlock Miniatures.

Lost Books - Second hand book supplier.

Osprey Books - Supplier of probably the most popular range of wargaming reference books.

Oxbow Books - Another book source.

Helion & Company Military Books - The name says it all.

Cambridge University Press - What it says.

Boydell and Brewer - UK based.

Oxford University Press - many useful publications.

Pen & Sword Books - a wide range of historical books.

Paul Meekins - second-hand and out of print books as well as new.

Ken  Trotman - Military history & wargaming books.

Monarch  Military Books - also stock figures.

Bookfinder - searches lots of places including other sites mentioned here.

Alibris - another wide search site.

Used Book  Search - pretty much what it says.

Biblio - yet another search site.

Byzantine  Books - site to get books on Byzantium from.

Hanshan Tang Books - excellent and friendly bookshop to get eastern information.