At this writing, one-month tourist visas, which cost US$20, are available at all border crossings listed and at the airports. You'll need two passport photos. 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 7:30-11:30 and 2-5.
Unfortunately, travelers report corruption at many border crossings. Cambodian authorities often will ask for a US$1 fee at the Laos border, or for 1,200 Thai baht or more (well above the legal US$20 fee for a tourist visa) at the Thailand crossings. Often, there's not much you can do to avoid this unless you speak Khmer.