Are you talking about the white slipper dessert available at the GF? I have heard of others getting it at the Castle, but don't remember anyone recently. Most order it from the GF bakery to have there at the GF - we ordered ours and had them at GF Cafe. Those have to be ordered at least 48 hours in advance, and are available just at the GF as far as I know - though you can inquire about having them served anywhere else and see what they say. We were told that they can't be delivered anyplace. They are delicious, btw.
Maybe there is another slipper dessert I'm not familiar with at CRT that you're referring to?
As far as just showing up, you might try mid afternoon or something, but it's a pretty popular place and they often don't take walk-ups anymore, esp not just for dessert, but you're always welcome to stop by and ask - never hurts! If you are set on going, you will want to try and snag a PS.
Hope that helps!