You get it in the Granite Cave in Dewford Town.
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HM 05 is flash
In Pokemon Platinum, Flash is a TM.
A HM is a move is a move that can be tought to as many Pokemon as you like but the Pokemon has to be able.
the HM flash is in the cave on Dewford town and Steve, the first man in there has it. All you have to do is talk to him but you do NOT have to battle him. Almost the only time you have to use flash is when you reach Ever Grande City. Shortened answer Talk to the fat guy in the entrance to the cave in Dewford.... :)
talk to Wallace