to play ds games, i would recommend this site.
http://www.peachds.com/?product=/r4isdhc-v2015/
they accept prepaid cards, so if you feel giving out your card info online is a bad idea, wait till christmas to get a giftcard or go down to your walmart/cvs/wallgreens and buy a $25 visa/mastercard/american express gift card, (after activation will be like $30), then register the card (through giftcard website), and buy, they will e-mail you telling you payment is processed and give you a tracking number (have to wait a couple days for payment to go through), so for that $30, you will get the r4 and a 16 gb micro sd card with card reader to connect to pc and you are good to go (need to go on internet to download firmware and games too of course), and you will still have 2 dollars on that card you can use next time you go to the 99 cent store or something, remember rule of thumb when using vista/mastercard giftcards online, you cannot use full amount and must leave 1 dollar unspent for credit card validation.
but you have a 3ds why not buy a card to play 3ds games? there are currently only 2 flashcards that play 3ds games the sky3ds and gateway, they are way more expensive but its a investment, anyways its your money gl.