Silver Ore

Silver sits in the underground and cavern layers in enough quantity that you rarely have to go looking for it on purpose. It also turns up in the small ore pockets that float in sky islands, which is worth remembering when you are already up there raiding chests.

Slimes drop it now and then on top of their usual loot, which is the part most people never notice. Very early on, killing every slime you walk past is a slow but completely free trickle of ore while you are doing something else.

Every world rolls one of the two and never both in the ground, so a tungsten world simply has no silver to find. They occupy the same rung of every recipe tier and the resulting gear is close enough that neither is a reason to reroll. If you specifically need the other bar, a second world or a friend is the route.