I can't find anything that says it's illegal. But, I know that FWC really frowns upon it. I do know that it is illegal from a commercial collecting perspective.
The only thing I could find in the marine ornamentals section of the FWC regulations was a reference to a 100 lb per day or 2 species limit for anything not previously listed.
Meaning, since sand was not specifically listed in the regulations, you could collect up to 100 pounds.
I personally have collected my own sand. But, the last two tanks I upgraded, I just used Caribsea live sand from the LFS.