I've read that some people go to Khao San and pay about 20 USD to a travel agency in order to obtain a tourist visa for Cambodia. That's ok with me, but I don't know the exact date when I'll enter Cambodia, because I will travel through Thailand and Laos before entering Cambodia. Is that a problem? Is any travel agency ok or would you recommend a particular one?
I should have mentioned straight away that I mean to enter Cambodia from Laos.
You can get a tourist visa on-arrival in Cambodia at all airports and most main land border points with Thailand, so you don't need to obtain one in advance. If you still want to get one in advance, the easiest way is to apply online for an 'e-visa', the details for which are listed on the Cambodian Ministry of Foreign Affairs website. Getting a visa in advance isn't necessary, just that you don't have to wait in a queue with the unwashed masses of other backpackers to get your visa. In my opinion, getting a visa in advance is not worth the extra fee you pay for processing, at all.