The Fabric Hierarchy
Cosplay fabric decisions cascade from the most important constraint downward. First: does the fabric need to hold structure or drape? Structured garments (armoured looks, fitted bodices, period silhouettes) need woven fabrics with body; draped garments (robes, capes, flowing dresses) need fluid fabrics with good drape. Getting this wrong produces a costume that looks permanently wrong regardless of construction quality.
Knit vs Woven
Woven fabrics (cotton, linen, silk, polyester satin, brocade) don't stretch, hold structure well, and are cut on the grain. Knit fabrics (jersey, spandex, athletic knit, velvet knit) stretch and recover, are forgiving of fit imperfections, and work well for close-fitting superhero suits, bodysuits, and any costume requiring significant movement. Most anime costumes that appear to have rigid forms are actually constructed in stretch fabrics.
By Costume Type
Armour and structured pieces: craft foam and Worbla, not fabric. Superhero suits: four-way stretch spandex or athletic knit. Fantasy gowns: duchess satin, taffeta, brocade, or chiffon depending on the silhouette. Anime school uniforms: a cotton-polyester blend in appropriate colours. Historical costumes: linen, wool, or period-appropriate reproductions. Character capes and cloaks: velvet, wool melton, or polyester fleece depending on budget.
Where to Buy
Fabric stores (Jo-Ann, Fabric.com) for standard materials. Online specialist retailers (Spandex House, Mood Fabrics) for performance fabrics and specialty materials. Thrift stores for quantity of inexpensive fabric for mockups and test cutting. eBay and Etsy for specific prints, brocades, and hard-to-find materials.