Visa Information » Cambodia
Unless you are from Indonesia, Laos, Malaysia, Myanmar, Phillipines, Singapore Thailand or Vietnam you need a visa. 

You may obtain your visa upon arrival at the two airports in Cambodia: Siem Reap and Phnom Penh, along with most border crossings. You will need US$20 cash for a tourist visa and US$25 for business. You will also need one passport size photo and your passport must be valid for six month from the date of entry. 

A visa can be issued upon arrival at the border checkpoints into Cambodia. 
Thailand: Cham Yeam (Koh Kong), Chorm (Oddar Meanchey), Daung (Kamrieng Battambang), Poi Pet (Banteay Meanchey), Prom (Pailin), O Smach (Oddar Meanchey). 
Vietnam: Bavet (Svay Rieng), Kaoam Samnor (Kandal Mekong), Phnom Den (Takeo), Trapaing Sre (Kratie). 
Laos: Dong Kralo (Stung Treng). 
There is also an entry at Sihanoukville if you arrive by sea. 

Cambodia visa information 

Cambodia embassies 

Cambodia has recently started an e-visa service which allows you to pay online using a credit card instead of handling cash at the airport. 

The e-visa is only for use at Phnom Penh International airport, Siem Reap International airport and some of the land borders. It costs five dollars more than paying cash. 

Cambodia e-visa