Meet with the production designer, director and key leads in your opera to begin a dialogue about appropriate costuming. Research the era of your opera and decide what elements will work best with the overall vision of the production.
Get precise measurements for all members of the opera company who will be wearing a costume. Write down the person's role, measurements, and if possible a rough sketch of what they will be wearing so searching for the right costume to buy will be easier.
Take two or three people with you on search for opera costumes to buy. Look in the phone book for costume shops in your area, and pay special attention to whether they provide stage costumes.
Call in advance to various costume shops and make an appointment to meet with someone regarding your specific needs. Mention your needs to the salesperson and provide them with as much information regarding your production as possible.
Work with your production's business coordinator to set up a purchase order or payment for your costumes. Bring in as many actors as possible for alterations on their costumes at least 2-3 months before the performance; you want to provide ample time to the alteration specialists.