Passports and Visas
One-month single-entry tourist visas, which cost $30, are available at all border crossings and at the airports. You may need a passport photo—if you don’t have one with you, it’s an added $5 to have it made there (no added wait; sometimes you might not even be asked for a photo). When crossing on the bus from Vietnam, bus operators will ask for a fee of $5 for helping "fix" your visa. This is not compulsory and you can arrange your own visa at the border, but it does make the crossing slightly less of a hassle. Don’t forget that you must have your visa for Vietnam in advance, and the dates must be relevant. If you are over or under the specified dates on your visa you will be charged on average $10 per day.
If you want to cross into Laos from Cambodia, you'll need to secure your visa to Laos in advance, as they are not available at the border. Border crossings are open daily 7:30 to 11:30 and 2 to 5. Unfortunately, travelers report corruption at many border crossings. Cambodian authorities often will ask for a $1 fee at the Laos border, or for Thai B1,200 or more (well above the legal $20 fee for a tourist visa) at the Thailand crossings. Ignore them and head straight to the border (a short walk from where the buses leave you) where you will find the appropriate offices and authorities—and in high season, several long lines. You need one completely blank page in your passport in order to get a tourist visa.