once you have the airship put the password "GODHAND" into it then goto mushroom rock road go north west as far as you can go and you find the god hand in a chest.
you get the mercury crest is in west sanubia desert.
the sigil is also easy, but takes about an hour...
goto west sanubia desert then go right from where you come into it, over the hill and you will see a stone with a cactus on it go over and press X then you will get a clue to where the first Cactaur is. theres 10 and you need to find them all in the desert and get a sphere from them.
goto
http://www.bellaonline.com/gaming/strategy_lib_ffxwalk_ur.html for more on how to find them.
when you found one you have to run up when it is turned away, then when it turns round stop moving and wait till he turns again. if you catch him you get a personal sphere if you dont you get a normal sphere but i cant see what difference it makes unless you get something if you get all 10 personal spheres. both types of sphere will clear the sandstorm when you have all 10 put into the stone, good luck.