The easiest place to farm this is to max your Garrison’s mine to level 3.  There are two node types, the regular True Iron Deposit and Rich True Iron Deposit.  However, you will definitely get far more Blackrock nodes in the mine.

For farming in the world, it can primarily be found in Nagrand (Draenor), Shadowmoon Valley (Draenor) and Shadowmoon Valley.

Do check the auction house for pricing on True Iron Ore if you are farming it for gold, the price is generally fairly low from Garrison farming.