If you want more Smithing Stones to upgrade your weapons in ELDEN RING, this guide will explain how to get the ability to buy infinite Smithing Stones
How to get +1 and +2
If you’ve visited the hub, you’ve likely seen the twin husks merchant off to the side, who also gives you the option to offer her “bell bearings.” This functions just like ashes with the shrine handmaid in Dark Souls 3; giving her bell bearings increases her shop with new items.
… In Liurnia of the Lakes, there is a cave off to the North East Side. Location below, Raya Lucaria Crystal Caves.
Fighting your way to the end and beating the boss rewards you with a smithing bell bearing to give to the Twin Maiden Husks. This unlocks infinite smithing stone (1) and (2) at her shop.
How to get +3 and +4
You need to find the Smithing-Stone Miner’s Bell Bearing to unlock the Smithing Stones 3 & 4 for purchase. To find this item make your way to the Atlas Plateau and visit the Sealed Tunnel named location to the southeast of the Outer Wall Phantom Tree Site of Grace.
To start hit the tunnel entrance to reveal a path going forward. Head forward past the first enemy and take your first right along the path blocked by some barrels, crates, and a wheelbarrow. Inside the room here there is a chest. Open the chest to get the Smithing-Stone Miner’s Bell Bearing.
Take the Smithing-Stone Miner’s Bell Bearing and head to the Roundtable Hold. Go to the Twin Maiden Husks and offer the Smithing-Stone Miner’s Bell Bearing to them. Doing this unlocks the Smithing Stones 3 & 4 for purchase from them. The prices are as follows:
Smithing Stone  – 2400.
Smithing Stone  – 3600.
These are the first set of major materials you can unlock from the twins. Once unlocked the sisters sell an infinite amount. The next few sets follow the same system. Find a bell, bring it to them, unlock the materials.
How to get +5 and +6
The third Smithing Bell Bearing comes up much later in the game, and this is where we get into SPOILER territory. It’s south of the Zamor Ruins site of grace, in the ruins. You’ll find a staircase with torches on either side leading underground. Open the door, then the chest, and that’s where it is.
How to get +7 and +8
The fourth and final Smithing Bell Bearing drops from the main story boss called Godskin Duo, in the Dragon Temple.