if u do pirating the merchant ships can carry a pretty large quantity of ingots tho the type is random if ur looking for just iron prolly better just to mine in fel
Specifically:
Pirates have Iron, Dull Copper, Shadow Iron, Copper, Bronze and Gold ingots (and ore which comes in piles equivalent to 7 ingots).
Merchants have Agapite, Verite and Valorite.
I've seen as little as 50-150 of each type (pirate), as much as 600 of each type (merchant), on one ship.
So, if you're looking for the high-end ingots, you'll get far better return on time invested hunting down the Merchant Tokuno and Gargoyle ships.
If you're looking for the lesser metals, unless you're hunting with a team of people to make the pirate captains go down quicker, you're better off mining for it (especially since it's easy to get Prospector Tools and Gargoyle Pickaxes now).
The most efficient time/gold cost means of getting iron ingots, though, still has to be searching Ilshenar's 10 random Gypsy camp spawn sites. If you're the first person there and find an Iron Worker NPC, iron ingots are 8 per for the first 1000, going up from there (and they despawn after a few hours, before the camp respawns in a different spot, resetting the prices again). Though, people have been making that harder to do ever since I pointed that out a decade or more ago, as there's a lot more competition for finding those NPCs now (especially with everything metal being crafted from iron first for reforging).