Cambodia Visa Service

All visitors, except citizens of most of ASEAN countries need a visa to enter Cambodia. The official price for a tourist visa is US$20, and US$25 for an business visa. Staff may try to charge more at some land border crossings: hold out for the official price, particularly at major crossings, but don't be upset if you have to pay US$1-2 extra.
Visas can be obtained at Cambodian Embassies or Consulates. Visas are also available "on arrival" at Cambodia international airports, all six international border crossings with Thailand, some international border crossings with Vietnam, and at the main border crossing with Laos.

Tourist visas: all are valid for one stay of up to 30 days. Those issued in advance expire 90 days after issue. In Phnom Penh (or elsewhere via agencies), tourist visas can be extended only once, allowing an additional 30 days at a cost of US$15.
Visa-E or Business visa – this is the best choice for those wishing to stay for over two months with multiple entries, as a business visa can be extended indefinitely (approximately US$145 per 6 month extension and US$290 per 12 month extension) and have multiple entry status when (and only when) extended. Most Phnom Penh travel agencies process the extensions. Foreign nationals of some countries from South Asia (including India) and Africa are recommended to apply for a Business visa at the Cambodian missions in their own countries as the conversion process from a Tourist visa to a Business visa within Cambodia can be expensive and annoyingly burdensome (~US$200 for conversion from Tourist visa to Business visa and another US$285 for a one year extension). There is always some more commission involved if you are travelling from a developing country to the range of $30-40. However, once you are in possession of a long-term Business Visa, travel into and out of the country is very convenient and painless.

(From wikitravel.org)