Research the show. Before looking anywhere for costumes, you have to know what to look for. View several versions of the musical your high school is producing and focus on the costuming. Use the Internet or the library to research the location or time period where the show is set.
Find a hook. Figure out what one element of the costuming is most important to help your young actors convey the message of their musical. Perhaps the time period is key to understanding the show. If the show is set in Victorian times, high collars are your hook. Keep that element in mind as you're looking for costumes.
Look for ready-made costumes. Depending on what time period you're depicting in your high school musical and how elaborate the costumes need to be, you may find it cheaper and easier to get ready-made clothing for costumes instead of hiring a seamstress to make the costumes or renting costumes specifically designed for your show.
Start with cheap items. Armed with a list of key pieces you need to costume your show, hit all the thrift stores in your area. You may be able to find puffy-sleeved dresses, narrow-lapelled suit jackets and other items you can alter to portray the era of your show. Be creative, and remember that clothes can be dyed different colors or taken apart and combined with other items to suit your needs. Taking the skirt off one ten-dollar thrift-store dress and attaching it to a fifteen-dollar dress with the right sleeves is still cheaper than buying a custom-made costume for your ball scene.