Short answer
Tin has no ore node in Graveyard Keeper 2. You make it at the Sluice Box, unlocked with the Bronze Forge technology: 3 Sand gives 2–3 Tin. Sand sits southwest of the Inquisition Base, below the western fishing spot next to the stream. Combine 3 Copper Nuggets, 1 Tin and Fuel at the Furnace to smelt Bronze Ingots.

The Town workshop
View full-size image ↗Official promotional screenshot · © Lazy Bear Games / tinyBuild · Official media kit · Material credits
Getting Tin from Sand
Research Bronze Forge in the Metallurgy tree, then build a Sluice Box. Each batch takes 3 Sand and returns 2 or 3 Tin.
Dig Sand southwest of the Inquisition Base, under the western fishing spot beside the stream. Fill your bag before you head home so the Sluice Box never idles.
Players looking for a tin vein in the Old Mine won't find one — guides agree there is no world node.
Turning tin into bronze
The common recipe is 3 Copper Nuggets + 1 Tin + 20 Fuel at a Furnace. Guides list 4 or 5 Bronze Ingots per batch, so check the in-game tooltip.
Copper comes from the vein on the cliff west of the Yard, near the pond where you learn to fish. Bronze Scrap can also be smelted at the Furnace as a backup.
Original summary of the bronze line that Tin feeds: Copper Nuggets and Tin go into the Furnace together. Not an in-game screen.
View full-size image ↗Original diagram · Graveyard Keeper 2 Wiki · editorial illustration · Original wiki diagram
Other uses
Tin is also an alchemy ingredient. Tin + Blueberry works for Explosive Dust, and Clay + Tin + Beeswax is one of the listed mixes on our alchemy pages.
Sources
Related pages
Official screenshot · © Lazy Bear Games / tinyBuildCraftingHow to Get Bronze Nails in Graveyard Keeper 2
Official screenshot · © Lazy Bear Games / tinyBuildCraftingGraveyard Keeper 2 Polished Bronze
Official screenshot · © Lazy Bear Games / tinyBuildCraftingGraveyard Keeper 2 Iron: Ore, Ingots and the Smithy
Official screenshot · © Lazy Bear Games / tinyBuildCraftingGraveyard Keeper 2 Fuel for the Kiln
Official screenshot · © Lazy Bear Games / tinyBuildCraftingGraveyard Keeper 2 Explosive Dust