Hi there, Kristine!
It is absolutely possible that you could get a walk-up appointment at the
Bibbidi Bobbidi Boutique. People do cancel or simply not show up; it is certainly not unheard-of for appointments to open on a daily basis.
There is not really a queue for the walk-ins. Essentially, you'll show up and be told yes or no for. Again, if someone is a no-show and does not call to cancel, that's kind of the luck of being in the right place at the right time.
Do not feel that you have waited too long to book! Cancellations happen. I would recommend calling at least once a day until your trip. You never know what those Fairy Godmothers-in-training might have available.
The number is (407) WDW-STYLE. Keep trying. I will definitely be sending pixie-dusted thoughts your way!
See you later, celebrator,
-Deborah B.