Where can I buy a Disney gift card, and where can I use it?

I was planning on getting a Disney gift card for my next trip, because I don't want to carry a lot of money around. Where can I buy one? I've seen them at Publix and other places but are there other stores you can get them at? Are they reloadable, and can you buy one and put as much as you want on it, or are there specific types? And where can you use them at Disney. I'm not so sure about the movie theater at Downtown Disney, but I know you can use them at the parks. Thanks for your help!