Depending on the city you will be near in Florida, you will probably find an art supply store that will meet your needs. If you want to order online, check the Dick Blick site that advertises here at Wet Canvas. You might also check out www.cheapjoes.com
for large canvas on rolls if they don't carry the size you need already stretched.
You won't have a problem with the regular sized canvases in Florida...I've lived there and painted there before and never had a big problem with the humidity.