The old 'Nintendo Points' cards are for Wii and DSi only. Only the newest cards, which have Wii, DSi and 3DS on them, can be redeemed in th 3DS eShop.
Yes, you can use the GameStop card to buy eShop credit which is an indirect way of adding money to your Nintendo Account
No. Because the eShop no longer uses Nintendo points, they will have their own set of prepaid cards.
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eBay or amazon
At Game Stop.
Nintendo eShop prepaid cards can be bought wherever you can buy Wii U or Nintendo 3DS games. Example of this include Future Shop, Best Buy and EB Games. It is also important to ensure that the card you are buying is an eShop prepaid card. Wii points or Nintendo DSi points cards cannot be redeemed on a Wii U. On the other hand, older eShop prepaid cards branded as "Nintendo 3DS" prepaid cards can be used on the Wii U eShop.
In order to add funds on the Nintendo eShop, you can either connect your credit card and withdraw cash from there, or you can buy a prepaid point card which can be redeemed by entering the pin afterwards.
The content will go on your SD card.
Many major retailers now sell these cards such as Gamestop, Walmart, Target, and Best Buy (those are the ones I can think of right of the top of my head). However, in the eShop if you want to add money, all you have to do it take it right from your bank account providing that you have a bank account and a debit card to go with it. It would save you a trip to the store.
To add funds to your Nintendo eShop account, you must either have a prepaid Nintendo eShop card or a valid credit card. After launching the eShop, swipe to the right or tap the menu button to find the "Add funds" button. Alternatively, if you're looking to buy a specific game, do not have enough in your eShop account, and are paying by credit card, you can browse to the game you want to buy, then tap "Tap here to purchase." When the eShop tells you that you do not have enough to buy it, choose to add funds to your account, select "credit card", then select "needed" to add exactly as much as you need to buy that game. Remember that your eShop account is tied to your 3DS and cannot be used on another system.
Gamestop gift cards can cost as much as the amount you put on the card so in other word a Gamestop gift card it the amount of money you put on the card
Gamestation sells them.