Isn't it the dress Meiling is using in Touhou Chireikiden ~ Cheating Detective Satori ?

It is, but I'm pretty sure she has a dress instead of a side slit.
As a side note, that's a pretty risque design for the series, somewhat on par with Byakuren's biker suit or Urumi's exposed midriff.

Meiling's outfit is incredibly lewd for an official Touhou story.