Secondhand shops can be full of useful costume pieces, but a little planning makes browsing more productive. Before you go, write down the silhouette, colors, and two or three key items your idea needs. Keep the list flexible: a long coat might become a detective's outer layer, a royal robe, or a space traveler garment depending on how you style it. Set a personal spending limit before shopping and focus on pieces you can realistically use.
Check garments for fit, comfortable movement, working closures, and visible damage. Look at seams, hems, lining, and care labels where available. A minor missing button may be easy to replace, while extensive repairs can take more time than expected. Consider how a garment can be altered or layered, but do not assume every change will be simple. If possible, try on the item with the sort of base layer you expect to wear.
Accessories are often the most versatile finds. Belts, scarves, hats, bags, and costume jewelry can change the character of ordinary clothing without permanent alteration. Wash or clean secondhand items according to their care instructions before wearing them, and check for loose parts that could snag. Keep a note of what you bought and the character ideas it might suit. That small inventory can help you build future costumes without starting every search from scratch.