Well, it's pretty hard but try migrating Pokemon that has a move Sunny Day from Pokemon Fire red, Leaf green, Ruby, Sapphire and Emerald. Or just trade with a friend with that move.
Forgive this answer. Though I've played this game 4 times with different Pokemon I forget where to find it.
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use a TM
By using Sunny day, Hail, Rain Dance. Moves that It can learn
Yes, the TM Sunny Day is in Pokemon Emerald. You can find it in a cave in the middle of the lake in Route 120 called Scorched Slab.
in the safari zone
You can find Sunny day (TM11) in Scorched Slab, this is a small cave found in Hoenn. It can be accessed by surfing in the north of Rout 120, next to Fortree City, in a indented lake.