You can't get it without first getting the National Pokedex. When you get the National Pokedex, go to Canalave City and go to the house next to the sailor. Then he'll be in bed. First talk to him, then his mother.
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in the I.N.N in the canalave city (you must have member card and the boy in the canalave city sick and you awaking the boy with lunar wing)
the sick boy in Pokemon diamond is in canalave city in the house on the very bottom left corner .x
In Canalave city, canalave city is next to jubulife city
If you were paying ANY attention to the gym leader in Canalave city, you should know its Canalave city.