Received my dstwo from digitopz.com,official distributor,cost me 37USD or so ,works perfectly on my 3DS V4.4.0. Maybe you can reference,or you can directly go to dstwo official " Where to buy",or you can go to this forum or shop temp,lost of store here you will find.
Though you need to e-mail them for their paypal info (you can use cards through paypal, most people do AFAIK).
I've heard that it doesn't work outside of the US, and I haven't heard anyone verify that it has worked outside of the US.
But US-based Visa gift cards can almost always be activated through a website instead of an automated phone system, in order to provide the debit account with a billing address, making it viable for use with PayPal.
I've received and spent dozens of Mastercard, Visa, and even American Express gift cards and used them this way.
Slightly off-topic, but I've also used PayPal invoices to bill small remaining balances to completely cash out my gift cards into PayPal balance- For example:
I got a $10 AmEx gift card as a rebate from a video card I bought, and spent something like $8.25. It left me with like $1.75 and it's not very easy to make a charge that small due to the $1 verification charge direct-to-card billing systems may use prior to authorizing an actual final amount. But I sent an invoice to an alternate email address and "paid" it without logging into paypal, giving myself the remaining funds before throwing the otherwise useless card away. I tend to leave $0.01 on the card to avoid having a decline that could lock up the card for 3-5 business days.