Even if it is a government-operated company, you still may face a usual visa scam when you are brought to a ‘visa agency’ and are forced to obtain your visa to Cambodia there – obviously at a higher rate. The official visa fee for a tourist visa is USD30 – that is the amount which you pay while obtaining your visa on arrival at the border (though at land borders you will be inevitably asked to pay some THB100-200 extra for ‘speeding the process up’ – it is not actually necessary to do so). Have one passport-sized photo to attach to your application.
If you want to avoid these annoying situation at the travel agency or at the border, apply for your Cambodian visa online through the official web-site of Ministry of Foreign Affaires and International Cooperation of Kingdom of Cambodia. The e-visa costs USD30 plus USD7 for processing your application. It is accepted at the major international borders of Cambodia, including Pnom Penh International Airport, Siem Reap International Airport, Poipet (Banteay Meanchey), Cham Yeam (Koh Kong) and Bavet (Svay Reing).
Most probably you will spend about an hour at the ‘tour agency’ as the majority of the passengers of the international bus usually seem unaware of this scam or just choose to pay more. It is absolutely useless to argue, so be patient.