From FFXI Wiki
Obtained from checking the ??? near the shipwreck at (H-9) of Valkurm Dunes during dust storms (The ??? will not be visible unless there is an active dust storm). Changing areas and checking the ??? while the storm is still active will yield another sunsand, as will waiting for another storm.
There's only a 1/3 chance of it being needed, also it's incredibly easy to obtain, even if it's the "toughest" of the three due to the required Earth weather.
