In each of the camping sites in the parks there are always stores where you can buy supply, groceries and souvenirs for your trip. Remember that the store will be closed around 6 or 7 PM.
What to buy: Food, water for the day and beer for the night :)
The Namibian and South Africa have the same rate and is interchangeable ! At least the SA currency and notes !
What to buy: African wooden sculptures
What to pay: The best is still the US dollar in travellers or cash.But dont pay with 100 dollar noted - They might be refused.
Tip : Petrol stations do not accept credit cards !!