How Do You Get Suicune In Pokemon Mystery Dungeon Explorers Of Sky?

You get it by getting a challenge letter from spinda's cafe(same goes for jirachi and moltres)
you can't get it from wonder mail I have all ready tried and I have the letter and it doesn't have wonder mail. I can give info however, he will challenge you at the trecherous waters b11f there are no restrictions and the difficulty is 5 star (800 points for completing it) the loot is unknow as well. I sugest that if your strong enough (I am a level 73 donphan with full IQ points) you go alone, but if you go with a friend make sure you get the finshing blow. Also the dungeon (treacherous waters) only makes you take in 8 items and no money. And he is most to have minions. Also you can get more info from looking it up on google or an official pokemon website.
Sorry I couldn't be of more help to you and good luck!

Its the same as mew two raikou entei and jirachi you need to receive a challenge letter from spindas cafe or enter the wonder mail s code. I don't know the Wonder mail for  suicune and raikou but if you do a search you will get the codes for jirachi mew two  and entei.   Also go to   www.serebii.net  to find out how to obtain other legendarys.