The only place that I know, and the only one you should probably be using, is the Sportsbook in the Naga World entertainment and casino building. I usually go to Naga World 2 and go to the basement area next to the food court and use those machines, but Naga World 1 has a sportsbook area as well next to the poker section on the upper floor.
The good thing about this sportsbook in this casino is that they offer customization options for your bets, and the odds are generally good. The bad thing is that the available bets at all times seem to be under some timed system, so sometimes I might have a bet in mind, but that specific game might not be there when I show up at the casino.
Personally, I think, even though you can be offline in Cambodia, the best place to bet is still online if the units you're wagering are small. But if you're wagering bigger units, and mostly doing straight bets, then at that point I would start exploring the offline options.
You can always check my recommended list of sportsbooks here.