Question

Where can you get a Shiny Stone in Platinum?

Answer 1

• Route 210
  > South of the pit between the climbable rocks (requires Rock Climb) (hidden)

• Route 228
  > At the end of a narrow bridge south of the pair of Pokémon Rangers

• Iron Island
  > In the last room

• Pokemon with the ability Pickup
  Between Levels 41-60 a 4% chance to find one. Between the levels of 61-100 a 10% chance to find one.

Source: Bulbapedia pages

Answer 2

There are only 4 ways to get a Shiny Stone in Platinum:

1. Iron Island B3F - On the ground located near the exit to the cave on this floor

2. Route 210 - Down a Rock Climb spot on a small dirt ledge, hidden on the final tile of the dirt ledge

3. Route 228 - From the Resort Area just head northwest and you'll see a small wooden plank above you that you can cross on the bicycle. This leads to an item on the ground on a raised platform, and it's a Shiny Stone.

4. Pickup (Level 61+) - 10% chance of getting a Shiny Stone with each pickup

This page has step-by-step tutorials on each location: https://guidestrats.com/pokemon-platinum-shiny-stones/

The easiest Shiny Stone to get is the one in Iron Island. But once you pick up all 3 of the first locations, the only way to then get repeatable Shiny Stones is to farm them via the Pickup ability. But if you just want an early Shiny Stone, the Iron Island location is ideal. Here's a screenshot of that item's location in Iron Island, although the link above has step-by-step screenshots to follow the whole way.

(Source: Guide Strats)