If a group of you are coming down and want to be as close to "guaranteed" on wind as possible, I would go to the
www.thekitehouse.com in Key West.
March is a great month, but you can still hit lulls any time of year, the flats are supposed to have thermals so even if the wind dies a little there is still a good breeze down there.
The Kitehouse can also set you up with accommodations, but you will want to call as soon as possible to make reservations because March is probably a busy month with spring break.
If staying in Key West is too expensive, there are plenty of rental houses between 5-20 miles north that a group of you could get for a good rate.
It really bites to plan a vacation and have no wind, your highest percentage of probability is hands down Key West.
So far this season Key West has had many more rideable days than the rest of Florida, even more than the rest of the keys.