Paladin Press is good. Probably the largest publisher of such material (at least here in the US).
Delta Press has some good stuff. Not sure if they are actually publishers as their name suggests, or if they are just a book store though.
http://www.deltapress.com/catalog/Also
www.nohoax.com has a nice selection of books.
I get most of my stuff in .pdf format for free from various sources though. Check around the different torrent sites.
