There are LOTS of threads on this, you can do a search if you want...
You can only order them through the Grand Floridian, but they will serve them at any of their restaurants, including their food court, tea, and room service. Just can't leave the grounds due to the ingredients and presentation.
You have to call the GF Bakery in advance and place your order. Then just make your PS wherever you wish, and when you arrive, mention it to your waiter/waitress.
We had a lovely lunch at the GF Cafe one afternoon and surprised my mom with the slippers. They are beautiful, delicious, and she was blown away. Seeing that she's a self professed Disney addict AND a CM that didn't even know they existed, it made it truly special for her.
They are $14 a piece now. Worth it only if it'll really blow someone away and if you'll eat it all. Otherwise, I don't think it's truly worth it... for most people, regular things at Disney are surprises enough =)
BTW- The Castle does not offer them, but I do think they have something served in an actual glass slipper, do some searches, you might find out more. But it's not the slipper dessert that has been pictured and talked about at length here.