The most regularly used currency in Cambodia is the USD. Only take $1, 5$, $10 and $20 for spending. Ensure that they are in good order with nothing written on them or even the slightest blemish or tear – they will not be accepted.
$100 bills should only be used to pay for your hotel or a donation to the charity.
If you want to use Cambodian Riel, it is available everywhere.
An extra bit of advice. Cash machines dispense USD. When using a cash machine always take an amount that is not an exact $100. IE: $90 or $190 etc. THis means that you will have smaller denominations. Do not expect a small bar to change large bills or a tuk tuk driver to have change of any description!