During 2006 year Cambodia started project of issuing its visa electronically. According to the actual information visa has validity 3 month started from the date of „approval“. You can use them for single entry only and for one month stay. More information you can get at official web page evisa.mfaic.gov.kh.
You can still get visa at embassies at your home country. It is also easy to get Cambodian visa on the way at South East Asia without any problems – Cambodian embassy is at Bangkok (Thailand), Vientiane (Laos), Ha Noi (Vietnam), consulate is also at Ho Chi Minh City (Vietnam).
It was also possible to get visa at the border crossings but it is not clear now if it is still possible or if you have to have e-Visa advanced. If you are planning to get visa right at the border, check this situation with nearest Cambodian embassy.