The earliest it can be obtained is upon beating the third gym. On Route 36, after you defeat Whitney, gain the SquirtBottle, and interact with the Sudowoodo blocking the route, you can find Schoolboy Alan near the routes end to the east. If you talk to him, you can get his phone number registered in your Pokegear. When he calls, he'll offer to give you a Fire Stone the next time you see him. When he calls is purely random, so it may take you a while, but that is the earliest available option if you're looking to get a Fire Stone before beating the E4.
Hope this helped!