In reply to LastBoyScout: don't order either Bhat or Riel in Europe. Dollars are widely accepted in Cambodia, change is normally given in dollars, too, unless it's a really small amount. Cash machines (ATMs) dispense dollars. I would just take a debit card with me.
In thailand, I always paid Bhat, and I got bhat from ATMs. Perhaps for really large purchases, I mean 200 USD or more, it would be different.
In general, I would just take my debit card, as I've always done.
Ordering "exotic" currencies in Europe is always very expensive due to the bid-offer spread on exchange rates.