I believe there are a few trainers that have Luminions, or you could battle a friend that has a Luminion, or you can evolve a Finneon.
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you have to see all the Pokemon in the pokedeks
Pokemon diamond
get it from a honey tree
combee. you can see one is slouther honey on a tree.
In Pokemon diamond many Pokemon can learn flash for eg. Electric Pokemon such as Pachurisu Pichu and Luxray all (eveloutions) and yeah just experiment and see what can learn flash!