Cambodia has a well-deserved reputation as backpacker central, so you can expect to meet fellow travelers just about everywhere. If you really want to plug into the local backpacker scene, opt to stay in a hostel instead of a hotel.
They often have bars or other common areas for socializing. While hostels generally have a reputation for being youth-based places, more and more people are staying at them now.
You can get a shared dorm or a private room. Even if you get a private room, you’ll still benefit from staying in the common areas and meeting plenty of people with whom you can travel.