Cambodian visas available at the border. This Mekong River border crossing is near the Vietnamese city of Chau Doc. In Ho Chi Minh City, travel agents can make transport arrangements to Chau Doc where you will boat up to the border, enter Cambodia and change to a speed boat that goes from there to Neak Luong. At Neak Luong, disembark and take a taxi on Route #1 to Phnom Penh( shared taxi: 10,000 riel). Allow about 5 hours for the trip from Chau Doc to PP.
If you'd prefer to buy a package ticket for the trip from Ho Chi Minh to PP instead of arranging all of the transport yourself, several travel agents now offer this option.