You can find it at the Galactic Warehouse. You'll be able to use Fly outside battles after you defeat Maylene.
at the city of the Pokemon league
fly
You can find it at the Galactic Warehouse in Veilstone City.
The HM Whirlpool is not obtainable in Pokemon Platinum, for there is no use for it during gameplay.
Dig is not a HM in Pokemon Platinum but a TM, it can be found in the ruin maniacs cave.
in a Pokemon center
Select the Hm hit use and select Pokemon
In Pokemon Platinum, Flash is a TM.
In Platinum, it is fly.
After you get back Dawn/Lucas's pokedex back go into the small warehouse and press A on the Item and it will be HM fly.
you can get it in pokemon platinum!
its not an HM anymore you have to find it.