One of the best and cheapest places to rent scooters in Cambodia is in Sihanoukeville where you can rent scooters for a $4 a day without license or insurance but have your passport handy as you will need to leave it with them until you return the scooter (you can also pay a deposit). Be aware though that driving without a Cambodian license is illegal. Then again, if you do get stopped by police you can give them 5 bucks and you are good to go again.
Scooter rental in Cambodia is super cheap..around $4 per day for 125cc semi auto scooter. The only thing I would say though is that if you don't have any experience in riding motorcycles I would advise hiring a bicycle or hop on a tuk-tuk instead.
By law, you cannot rent a scooter in Siem Reap. Also, if you are not supplied with a helmet, do buy one as they're cheap (around $20). And, definitely make sure your insurance covers riding scooters abroad because getting airlifted to the nearest hospital is expensive!