It used to be that only locals could buy a SIM card for mobile phones. Tourists used to ask any local (e.g., their moto driver) to go into the shop and sign for the SIM card with their IDs.
Nowadays, however, SIM cards can be easily bought in telecom shops in Sihanoukville. There are three service providers to choose from: Samart, MobiTel, and Shinawatra.
A word of caution, however: mobile phone thefts are widely reported in Sihanoukville, especially in touristy areas like beach resorts and bars. When frequenting these places, it is always recommended to observe safety common sense by keeping an eye on important belongings.