In order to evolve Eevee into a Flareon you will need a Fire Stone. While this seems something of a no brainer it can sometimes be difficult knowing where to find one. In some games they're pretty easy to find, but in other games it can require a bit of time and effort. To hopefully make things easier here are all locations on where you can find yourself a Fire Stone. Keep in mind that not all locations listed are guaranteed to give you a Fire Stone so keep trying.
Red/Blue/Yellow: Can buy one at the Celadon Department Store in Celadon City.
Gold/Silver/Crystal: Can receive one from Bill's grandfather on Route 25, can receive one through Mystery Gift, or can receive one from Schoolboy Alan on Route 36 (Crystal only).
Ruby/Sapphire/Emerald: Can receive one from the Diving Treasure Hunter on Route 124 by turning in Red Shards or find one on the Fiery Path (requires Strength).
FireRed/LeafGreen: Can buy one at the Celadon Department Store in Celadon City or find one at Mt. Ember at the Sevii Islands.
XD: Can receive one as a gift from Perr in Gateon Port or can find one on the S.S. Libra.
Diamond/Pearl/Platinum: Can be found in The Underground, can receive one from Mr. Fuego after speaking to him for the first time at the Fuego Ironworks or somewhere in the building, can receive one as a held item on Vulpix if trading with Haley by giving her a wanted Pokémon from the My Pokémon Ranch game, can be found at Solaceon Ruins (hidden; Platinum only) or find at Stark Mountain (hidden; requires Rock Smash; Platinum only).
HeartGold/SoulSilver: Can receive one from Bill's grandfather on Route 25, can receive one from Schoolboy Alan on Route 36, can receive one by winning first place in the Bug-Catching Contest or can buy one at the Pokéathlon Dome on Sunday, Tuesday and Thursday.
Black/White: Can be found randomly in Dust Clouds in caves, can be received from a man on the pier to the south of the Castelia City entrance, can find one at the Desert Resort or can be bought in Black City (Black only) from Vincent.
Black 2/White 2: Can be found randomly in Dust Clouds in caves, can find one in Lentimas Town, can find one at the Desert Resort, can be bought at the Battle Subway in Nimbasa City, can be bought at the Pokémon World Tournament south of Driftveil City, can be bought in Black City (Black 2 only) at Store 2, and can receive at Join Avenue from the Antique Shop if bought a stone.
X/Y: Can be bought in Lumiose City at the Stone Emporium, can be found on Route 9, can be found Route 13 near the central building on the east side (hidden), can receive one from battling Inver on Route 18 if score ranked 7-9, can find one from spiky rocks in battles by using Hyper Voice or Rock Slide, can receive one from Super Secret Training by completing The Fire Stone Cup Begins! in Super Training, or can buy one from the Pokémilage Club by completing level 1 of Balloon Popping.
Omega Ruby/Alpha Sapphire: Can receive one from the Diving Treasure Hunter on Route 124 by turning in Red Shards, can find on the Fiery Path (requires Strength), can receive from the Inverse Battle Shop in Mauville City if score ranked between 7-9, can receive one from Super Secret Training by completing The Fire Stone Cup Begins! in Super Training or can receive one from one of your Super Secret Base pals when asking them to pick up Stones.