Shiny Leaves are rare to find.
But I found out how to quickly get them, thanks to some resourses.
Keep talking to your Pokemon while you're in the grass in the routes listed.
If it finds one, it will start out with a jump, an exclamation point, dance around, jump twice, and make a musical note.
When you find one, go to Lyra/Ethan and they'll tell you about the leaves.
You have to find five leaves to get the Leaf Crown for your Pokemon.
Once you have all five, talk to Lyra/Ethan again and they'll turn it into a Leaf Crown for your Pokemon.
You'll also get a star for your Trainer ID (only applies once).
Find your Pokemon's natures and routes to find the leaves here.
Spontaneous natured
Natures: Hasty, Mild, Quirky, Rash
First leaf: Route 44
Second leaf: Route 39
Third leaf: Route 26
Fourth leaf: Route 18
Fifth leaf: Route 11
Carefree natured
Natures: Calm, Gentle, Lax, Relaxed
First leaf: Route 32
Second leaf: Route 35 or Route 48
Third leaf: Route 2 or Route 28
Fourth leaf: Route 4
Fifth leaf: Route 6
Energenic natured
Natures: Impish, Jolly, Naive, Naughty, Sassy
First leaf: Route 31
Second leaf: Route 34 or Route 47 (Flying-types cannot find one on this route)
Third leaf: Route 1
Fourth leaf: Route 3 or Route 18
Fifth leaf: Route 5 or Route 12
Careful natured
Natures: Careful, Docile, Modest, Quiet, Serious
First leaf: Route 33 (Water-, Fire-, Ground-, and Rock-types cannot find one on this route)
Second leaf: Route 36
Third leaf: Route 45
Fourth leaf: Route 7
Fifth leaf: Route 8, Route 13, or Route 14
Brave natured
Natures: Adamant, Bold, Brave, Hardy
First leaf: Route 42
Second leaf: Route 37
Third leaf: Route 22 or Route 46
Fourth leaf: Route 15
Fifth leaf: Route 9 or Route 24
Lonely natured
Natures: Bashful, Lonely, Timid
First leaf: Route 43
Second leaf: Route 38
Third leaf: Route 27
Fourth leaf: Route 16
Fifth leaf: Route 10 or Route 25
NOTE:
You cannot find a leaf in the same route of where you found it.
Chances of finding a leaf grow shorter the more times you find one with your Pokemon, so continue to talk-spam with it.
Good luck!~