when are we going to meet the gensokyo tailor theres no way reimu can afford a new outfit every other week
The "Gensokyo Tailor" is Rinnosuke of Kourindou, and chances are Reimu just borrows his stuff whenever she needs a non-shrine-maiden-like disguise.
Maybe she pays a rental fee, but I strongly doubt it.
It's not a new outfit, at any rate. This style was
very fashionable in the 1920s. Young women wearing it were known as "flappers", at least in the US. Western style dresses were also a big deal in Taisho era Japan.
Reimu might even have worn it herself before. In one of the good endings to PCB, she attends a drinking party at Hakugyokurou wearing a hat much like this one.