The same place you get it in platinum.
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you can get an electric next to the wind works
it is in the dimiond castle in level 17
electric ghost dark and rock. =)
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!
one strong electric Pokemon is a luxray Or catching a Pikachu in the Trophy Garden on Route 212 and evolving it with a ThunderStone into Raichu