>>49679704
Go to anime conventions regularly and talk to some of the girls. Just don't come off as a creep only looking for sex. Put in your head that most people are there for friendship and selfies, not looking for a date.
The girl doesn't have to be into Touhou per se, like it could be a Pokemon or Sailor Moon cosplayer, but if you're lucky and the girl likes you then she might cosplay as Remilia for you in the future, once you become friends.