Most travelers don't need to apply for a visa from an embassy or consulate. E-visas can be obtained at
this Website (https://www.evisa.gov.kh/) or from a travel agent in advance of the trip.
You can even get a visa on arrival at Phnom Penh Airport (takes about 20-25 minutes). For the latter, you need to bring two passport photos with you and USD $30 in cash.