If you head over to Steam’s Summer Sale (ends July 5th) you have a chance to complete quests and collect special Sticker Books. One of the sticker available for you to collect is a special fairy sticker from The Sims 3.
Once you log into your account, head over to this page for a list of quests required to collect these special sticker books. The quest that granted me this special sticker was the “Explore popular tags” quest, but according to the FAQ, stickers are assigned randomly.
❗ FROM THE FAQ: Once the sale ends, all stickers and sticker packs will expire. Make sure to apply all of your stickers by July 5th @ 10am PST.