At the Veilstone department store (TM90).
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There is no HM Flash on Pokemon Diamond, I personally think its been replaced by HM05 Defog.
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!
You can buy TM70 Flash at the Veilstone Department Store.
the hiker is in mt.cornet
Pokemon diamond