Do NOT use the E-Visa system. LOTS of problems reported with it!
You can get a visa on arrival in Siem Reap. You will need at least one passport-sized photo with you. A tourist visa is $25 and can be renewed once. A regular visa is $20. A business visa is $25 (I think) and can be renewed indefinitely.
Do note that visa prices differ at different entrance points. almost 6 years ago