Be very wary of purchasing these gift codes. They are frequently purchased with stolen credit cards. Netflix cancels the subscription when the credit card payment for the gift code is reversed.
These subscriptions are not purchased with stolen credit cards.
My subscriptions come from when Netflix partners with many different other brands to offer limited time promotions where netflix memberships are bundled with products like LG Smart TV's or Google Chromecasts. The people I get the codes from either get the codes from the people who buy the products and already have netflix/don't want it; or they buy the products, get the code, and sell the product on ebay or wherever.
None of my subscriptions have ever been canceled by Netflix.
If you have any other concerns let me know.