The issue is that your credit or debit card credentials must be associated with the same country where you reside.
"Although you can browse the iTunes Store in any country without being signed in, you can only purchase content from the iTunes Store for your own country. This is enforced via the billing address associated with your credit card or other payment method that you use with the iTunes Store, rather than your actual geographic location."
Keep in mind the iTunes and Mac App Store are one in the same.
It's not a matter of a distributor, it's a matter of digital rights managment.
Currently there isn't a Cambodia iTunes Store or Mac App Store.
I don't see Cambodia on the international Apple Store either > The Apple Store - International