Instructions
Mt. Moon
Get to Mt. Moon by walking east from Pewter City on Route 3. You can also access Mt. Moon's alternative entrance if you walk west on Route 4. If this is your first visit to Mt. Moon, your rival appears.
Battle and defeat your rival to gain access to the rest of the caves. Walk around inside Mt. Moon until a Clefairy appears.
Damage Clefairy slightly, but do not knock it out. Clefairy has weak physical defense and is easily knocked out. Capture Clefairy with any Poke Ball.
Safari Zone
Fly to Cianwood City. Head west to get to the Safari Zone. Once inside, pay the cashier the $500 entry fee. You get 30 Safari Balls.
Head to the Meadow area. Walk around in the bushes. Clefairy is one of several rare Pokemon that can appear.
Toss a Safari Ball at Clefairy to capture it. Do not try to feed or distract it with bait. Doing so gives the Clefairy a chance to run away.