But guess what? We finally found an online store that that sell Nintendo 3ds Prepaid Cards! It's about $7.90 more than the $20 original price but if you plan on buying more than one game on the eShop then this should be OK. You can check it right here.
Please take note that the prepaid card is compatible with Nintendo 3DS US version only. Also, if you live outside the US then you can set your 3DS eShop account to a state that doesn't charge taxes so you'll have a lot more value for your money.