When you mine the iron commodity there is a chance you will also collect Pure Iron as a consumable, this chance is higher when using ships such as the Barge. Therefore you should search for systems that have the iron commodity.
To check for any systems you have already discovered that have iron, open the cargo window and select search. From there use the "commodities to mine" group and change the drop down to iron. If no systems are displayed you will need to explore to find new systems, if systems are displayed use the engage option to travel there and start mining iron.
To explore for new systems open the helm window, select sublight and then with the explore group choose a direction to explore in (positive or negative). Your ship will travel in that direction and stop automatically when a system you have not yet discovered is discovered.