Links to websites selling Cambodian stamps and related philatelic items:

Ministry of Posts and Telecommunications, Royal Government, Kingdom of Cambodia: http://www.mptc.gov.kh/

CAMBODIA PHILATELY: A blog devoted to the fascinating post-1979 Cambodian philately: http://cambodiaphilately.blogspot.com/

George C. Baxley PO Box 807, Alamogordo, NM 88311, USA: Perforate (Regular) Issues, Imperforate Issues, Die Proofs & Deluxe Sheets, Covers/Stationery, Unlisted Sets: http://www.baxleystamps.com/camb.htm

Tibetan Post:
Stamps: http://www.tibetanpost.com/tpost.php?op=subsection&id=77
Philatelic History: http://www.tibetanpost.com/tpost.php?op=subsection&id=37
Postcards: http://www.tibetanpost.com/tpost.php?op=subsection&id=61

Hunter Stamps, 220 N. Main, Colfax WA, 99111, USA
http://www.hunterstamps.com/stamps-country.php?countryCode=CAM

Rush Stamps (Retail) Ltd, P.O. Box 1, Lyndhurst, Hants, SO43 7PP, UK
http://www.rushstamps.co.uk/foreign/cambodia.htm
info@rushstamps.co.uk

All The World Stamps - the online stamp catalogue of Stanley Gibbons:
http://www.allworldstamps.com/results.asp?cid=45&sid=978

Herrick Stamp, NY, USA:
http://www.herrickstamp.com/country_stamps/camb/cambodia_stamps/

United States Stamps for Sale – but they also sell stamps from many other countries, including Cambodia! http://unitedstatesstampsforsale.com/cambodia.htm

asian-stamps.com
An online stamp listings categorization project — serving stamp collectors and online buyers.  The Cambodian stamps are taken from eBay's listings — extensively categorized and making it easy to find Cambodian stamps.
http://www.asian-stamps.com/index.php?c=cambodia