The gift cards can't be spent in the park. You have to exchange them at Guest Services for Universal dollars. Then the Universal dollars can be used in the park. The gift cards don't work like Visa gift cards. I'm not sure who sells them but, honestly, it is more trouble than it is worth. Instead, you can stop at Guest Services and purchase the Universal bucks themselves rather than buying a gift card and exchanging it.
If you still want a gift card, I imagine that someone here knows where to get them. Sorry, I wasn't too helpful.