It's not a TM in gen V so your best bet is a pokemon that knows by level up or follow this guide to the move tutor (attached)
You get the Iron Tail TM by going up the waterfall in Meteor Falls and then going up a ladder.
TM 23 Iron Tail can be found in the Iron Mountain.
TM 23 - Iron Tail can be found on Iron Island.
After beating Jasmine, she will give you TM Iron Tail.
Iron Tail isn't a TM in the game.
iron island
In the Pokemon game corner
Defeat Jasmine a olivine city.then youll get iron tail.
jump on a bike and eat its tail
get TM23 (iron tail) in iron mountain
iron tail
It'll need to be hatched in a Egg if one of the parents know Iron Tail. I don't know how good a Pikachu (which knows Iron Tail) would do with Arcanine. Haha. But if you can keep finding Pokemon with a Iron Tail move, (make sure the Pokemon is male) you can breed it with a female Arcanine.