you can apply for tourism visa at cambodia embassy in singapore, or alternatively, you can apply upon arrival at any international airport in cambodia. but this often wastes your time if you're not willing to pay tips. i suggest you get your visa in advance to save your time in cambodia. silk air provides flights to phnom pehn and directly to siem reap from singapore. plenty of info online. google it! one place might be talesofasia.com if you don't have lonely planet yet.