After you defeated Mayleene your rival will be waiting for you and Lucus/Dawn will tell you that Team Galatic has his(Lucas)/ her (Dawn) PokeDex go after your friend and help him/her out. After doing so Looker will be having a little chat with you to and follow Looker in that hideout and you will see a PokeBall, that PokeBall contains HM 02 FLY.
You can find it at the Galactic Warehouse in Veilstone City.
its a hm and its called strength
You can just reteach it since HM are reusable.
In platinum you don't get a HM in the Great Marsh. The HM that is there in diamond in pearl, HM05 Defog, has been moved to the Solaceon Ruins
Bianca gives it to you in Driftveil City after you beat her in a battle. :D
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.
You can find it at the Galactic Warehouse. You'll be able to use Fly outside battles after you defeat Maylene.
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!