Visa to Cambodia can be applied at ports of entry whether you come by air or by land.
Travelers are required to show your phhoto (3x4cm) and their valid passport. The Government Immigration distributes application forms upon arrival.
If you arrive by air at Phnompenh International airport or Siem Reap International airport, it is so simple and just take you few minutes to get visa done. The fee is $30 USD for a 30 day visa (Cash only).
Obtaining a Cambodian visa is also possible at the Poit pet” border gate with Thailand (When you drive in from Thailand).
From Vietnam you could pass to Cambodia via few border gates: Vinh Suong at Chau Doc (Mostly people come by boat), Xa Xia at Ha Tien, or Moc Bai at Tay Ninh when you drive from Saigon. The Visa fee $33 USD Single entry.
More: Vietnam visa