how can i buy app or iTunes card with credit card in Cambodia?

As fas as I know iTunes gift cards aren't available in Cambodia (and as iTunes gift cards are country-specific you won't be able to use one issued in a different country). For your credit card, to be accepted by iTunes it needs to be registered to the same Cambodian address as you have on your iTunes account and was issued by a Cambodian bank, if it is then you should be able to add it to your account : Change or remove your Apple ID payment information in the iTunes Store - Apple Support (Visa shows as a payment method ?)