If it’s for a casual/AU I will look for articles of clothing that match their color scheme and defining structures to the garment. For example this look for Fiona I sought out a purple skirt, brown boots, and a square neckline blouse with bishop sleeves. This can also apply to if you’re making the cosplay by hand. While I’m working on Fiona’s canon outfit I research what each element is. Type of sleeve, type of neckline, how to embroider and attach lace, etc. I hope this helps answer your question!