You can find sewing patterns that will work for many costumes with minor modifications, and some creators even make cosplay-specific patterns to sell on Etsy or share with others in the cosplay community.
There are really only two main ways to get a cosplay: buying or making. Many cosplayers–myself included–spend hours designing and building what we wear. There are even online communities created to support the “build” of different types of cosplay, including Facebook groups for building your own custom Mandalorian. Thanks to said groups, YouTube tutorials, and premade sewing patterns, I’ve been able to puzzle my way through most of what I need. You can find sewing patterns that will work for many costumes with minor modifications, and some creators even make cosplay-specific patterns to sell on Etsy or share with others in the cosplay community. My Alicent Hightower cosplay (pictured below at my first-ever cosplay competition at GalaxyCon Richmond 2023) started off as a medieval dress pattern that I found at a thrift store and got to the finished product through tweaks, draping, and several try-ons.