Costumes is limited quantity, so you have to make a reservation.
Seems some rent shops you can also make a reservation with e-mail, or form.
This is the "Nebuta fes" official site, read all first with Google translate,https://www.nebuta.or.jp/join/
but could not jump to the site of shops fr page translated by G-translate,
so, to open the shop sites, use below original url.https://www.nebuta.or.jp/join/wear.html
If you have something of other Qs, it may be faster to ask questions
via "Aomori Tourism and Convention Association"
than the official "Aomori Nebura" fes office site.http://www.atca.info/atca_language/index_en.html
(jump to page bottom, send e-mail)
Also, both organizations have FB, but do not know about they will answer yr Qs on FB.https://www.facebook.com/aomorinebuta/https://www.facebook.com/atcainfo/
Costume is not only Yukata.
If you participate for more than two consecutive days or in the future,
it would be better to buy costumes as full set or part of.
Also you can be diverted or self made for most parts of costumes,
just wearing "BLACK yukata (called KARASU = crow)" becomes a violation.http://jongjong2323.com/wp-content/uploads/2017/03/6546.jpghttp://www.pomit.jp/%E7%89%B9%E9%9B%86%E4%B8%80%E8%A6%A7/%E9%9D%92%E6%...