It is possible to purchase a Subway gift card either online or at an authorized Subway gift card retail store. Pick the amount you want on the gift card, and pay for the card with cash, debit or credit. As of 2015, you can add between $5 and $500.
Subway sells its gift cards in its over 37,000 restaurants around the world. Visit your local Subway, and ask for a gift card from the cashier. He proceeds to question you on the basics: how much to put on the card, payment method and design.
Walmart, Walgreens and CVS sell Subway cards in their retail stores. Walmart also sells these cards online. Discounted Subway gift cards are available online from Amazon.com and eBay.