Most guys go to the girlie bars. Several hundred in Phnom Penh, mainly on Street 104, Street 130, Street 136. Few more scattered around. Cyrcee is a popular one for guaranteed sex:
https://www.facebook.com/profile.php...5&locale=fy_NL
In Siem Reap not much in the way of those bars. You can use Tinder or cruise Walking Street at night. Might be 25-50 women total in the city looking for white guys.
There are tons of local karaoke places and that's where you find the hottest girls. Though no one speaks English and you might not be able to figure it out without experience.