Backlot's Trophy Garden only shuffles in national dex mons in the postgame, when you have the national dex. You're still rolling the dice for Cleffa or Clefairy showing up that day, but the pool is definitely smaller.I wanted to do something like this
But it's hard, because Clefairy is non realistic, it should appear only in that shitty garden and that garden has rare-non-regional mons that change each day, also the grand underground doesn't seem to have cleff.
Another option was Slowbro instead of Clefable, but it seems Slowpoke is post-national dex? :S
you can get Cleffa on the first floor of Mt Coronet (Clefairy is available at higher floors but those are only available around the time yo uhave to climb the mountain for the climax, i think?), however.