Again, my experience has only been with bridesmaid dresses where unless they have the size and color they want, you'll have to wait while they order it, but the impression I got is that you can walk in and buy it if you want. You might want to call to make sure, but I think that's how it works- the friend whose wedding I was in ended up just walking in and buying her dress after finding it. She had to have it altered, which took a while, but if it fits, I think you could just walk out with it same day.

Yeah, David's Bridal is kind of mass-market wedding stuff, but damn if they don't have their shit together to get stuff done.