Doubel check your 3ds system version is very important. And as to where to buy the gateway 3ds card, i got mine from r4dpepot site :
http://www.r4depot.com/16-gateway-3ds-flashcart-play-3ds-games-on-3ds-xl-and-3ds-console.html They are the on the gateway 3ds official reseller list, and i used to buy r4i card from them. So i choose them first.They delivered within 5 business days to my house in USA with a tracking number. The customer service response time is quick and helpful. You can consider them, or you can also email to them to check.