The easiest way to get Amazon e-gift cards in Nigeria is through NOSH.
On the app or web app go to Buy Gift Cards, pick the Country of the e-gift card (e.g., United States), choose the brand of the e-gift card (i.e Amazon), enter the amount, quantity and complete your purchase. You will receive the e-gift card immediately.
To use your Amazon e-gift card simply log into your Amazon account, go to Redeem Gift Card, and enter the code you received from Nosh. It will be added to your Amazon balance and you can then use it to make purchases.