Anonymous donations from men poured in, and in just three months they had pledged the 2,500 pounds needed.
"It was an amazing feeling when I logged on and found out I'd raised enough," The Sun quoted Karen, as saying.
Since giving birth to daughter Kasey three years ago, married Karen had dreamed of bigger boobs.
But she admitted: "It wasn't something I thought too seriously about until I found out I could have them done for free."
A TV show told her about website freeimplants.com.
Women create profiles and men can donate for a minimum 60p a time.
She had the op last month, boosting her from a 34B to 34DD, in Prague, Czech Republic.
Karen, of Accrington, Lancs, said: "I'm really pleased with how it turned out. I went for a natural look and that's what I got."