"Grease" showcases 1950s looks, so begin a serious costume hunt in thrift stores. Black leather, or pleather, which is faux leather, and windbreakers are essential---keep an eye out for a windbreaker that could be dyed black. Wide skirts---the poodle variety, minus the poodle appliques---will add authenticity. The cardigans from sweater sets can be paired with gathered skirts and matching wide belts. A long skirt can donate its deep hem to cover cardboard for a matching wide belt. Don't forget to check the shoe bin for high-heeled backless mules that can be painted or dyed for stage.
Shop neighbors' and grandmas' closets, too. Put the word out for old Jackie-style princess dresses, pillbox hats, vintage cheerleader outfits, capri pants and tea-length full-skirted prom dresses. Collect anything pink you can find. Pink clothes can be remade for the girls in the Pink Ladies clique. Parents and grandparents may be happy to empty the attic for a good cause, and you may find some real gems to add authenticity to the performance.
"Grease" is a perennial favorite for Halloween costumes. You can often purchase inexpensive used costumes from online sellers, or get them for free from people just giving away useless stuff. Hunt for costumes you can embellish so they look slightly more substantial. Add a crinoline to a poodle skirt to fluff it out. Replace a tacky polyester Peter Pan collar with a more substantial felt or cotton Peter Pan collar. Double-stitch dance pants to reinforce them so they don't split in the middle of the choreography.
Olivia Newton-John supplied the backless mules and tough-girl jacket she wore in the movie from her own wardrobe. Check with the cast to see who has black stretch capris for gym workouts or black jeans that just need a wide belt. Those are perfect for key scenes. Cut up some T-shirts for off-the-shoulder numbers and use a white sweatshirt with felt appliqué glued on the front for a cheerleader sweater. See who has white Keds kicking around the house before you spend the costume budget at the discount shoe emporium.