They can be randomly found (though many respawn in the same place on a 15-20 minute cycle), planted with a Shining Vein Seed, or mined from the Excavation Site.
It costs 10 labor to mine an iron vein, which will produce 3-4x
Iron Ore and 0-4x
There's also a chance, - about 15-20%, but influenced by your mining proficiency, that a fortuna vein will appear after you have mined an iron vein. It costs 20 labor to mine a fortuna vein.
As fortuna veins seem to be the only way to obtain ores for more exotic metals - copper, silver, gold etc... - it is likely that iron ore itself will end up being regarded as a useful by-product of mining for those metals (and therefor fairly cheap) rather than as a valuable asset in its own right.