No problem - you can book a bus ticket online here, from Saigon to Cambodia - either Phnom Penh or Siem Reap.
Our tickets are accepted as proof of onward travel by airlines and Vietnam immigration, which means if you're arriving in Vietnam without a visa, and you're from one of the countries that do not need a pre-arranged visa, you will have no problems proving to airport staff and immigration officials that you have onward travel arranged. This is a requirement for anybody arriving without a visa.
Nationals of certain countries may visit Vietnam without a visa (but will require an onward ticket) - check which countries this applies to with your local Vietnamese embassy/consulate before booking travel.
Click here to book your onward ticket - Vietnam (Saigon) to Cambodia (Phnom Penh)