So, where do I find the Fire Stone in the earliest and easiest way?
The Solaceon Ruins is definitely the earliest Fire Stone you can get (also the earliest Water Stone you can get too). It's also really simple: enter the Ruins and take the very top staircase on your left. Then interact with the rock in the center of the room and you'll get the Fire Stone as a hidden item. The next earliest spot would be Fuego Ironworks where the Fire Stone there is locked behind Surf.
Source: https://guidestrats.com/pokemon-platinum-fire-stones/
