I've been studying in Shinjuku for a few months and I only have a week left before leaving, but I spontaneously remembered that you visited recently as well. I was wondering if you found any cool cosplay stores while you were here? Other than the top floor Akiba shop, which is the only wig + cosplay store I've stumbled across so far, I only really see occasional wig stores (●´∀`●)
OKAY COSPLAY POWER SHOP TIME. First place you wanna hit is the colloquially named "Otome Road" in Ikebukuro across from Sunshine city. Aside from Assist Wig, the entire 6 floor Animate on the corner is dedicated to cosplay stuff (there's a legit Animate a few blocks down with a MAPLE wigs across the street). There's a Cospa outlet, Lashinbang, lots of lightly used resale items, and even a cosplay photo studio on the top two floors! Keep going down the road past the doujin shops and one of the K-Books is dedicated to new and used cosplay. Next, there's the Nakano Broadway Shopping Arcade. The 2nd and 3rd floors are a goldmine of all sorts of merchandise, but there are also two (or three...?) cosplay shops. One has a ton of resin accessories like gems and brooches along with wigs and costumes.
There's also the famous "Tomato" chain of fabric stores in the fabric district, but you probably don't want to lug fabric home I'm guessing.
That's all from memory so some info might be slightly off, but Ikebukuro is definitely your best bet! Publishing for future reference aha...