You can get this on arrival but it will slow down the entry processing because you have get the visa from a separate immigration counter first before you proceed to the actual immigration officers for border entry processing.
Sometimes visa on arrival can get busy so you may have to line up with many other people to get your visa.
It's easier to apply online for a eVisa before you travel and then just proceed straight to immigration processing on arrival..
The eVisa is for a single entry up to 30 days entry into Cambodia .
And it's valid for 3 months so you can use it to enter anytime during a 3 month period....
Depends on your passport country but most require a visa but these can be obtained on arrival or before you travel via Email through the Cambodia government visa website..
It depends on your passport country and what your visa requirements are (if any) to enter Thailand..
If you are from 1 of the 93 countries that now get visa exemption entry then no payment is required unless you apply for a particular visa before travel for longer than your 60 day excemption entry period.