If you’re looking for floor length prom dresses, the best way to get one would be to have a dress custom made to your measurements. Some of the tall stores have floor length dresses, but they may not be as fancy as you would like. Prom dresses nowadays show a little too much skin for my taste (I wish my daughter would try to show that much skin).
There are still some nice ones out there that your daughter would be comfortable in without appearing too prudish.
Model Atelier has some of the most beautiful dresses for prom and formals. This is a store by a tall girl.
ASOS Tall has a few options for you.
Obviously Long Tall Sally will have a formal dress in the appropriate length.
If you want to have a dress custom made to your size, eShakti is a good place to try out.
If you prefer to have a tulle skirt, Rita Phil will custom make a skirt according to your measurements.
I also have a post of skirts that may be long enough for your tall daughter. Go check that out too.