Decide whether you need the real thing or if a limited edition replica will do. Replicas are made by licensed companies that sell to the public. Many are limited in number and more details are put into the piece.
Search eBay by the performer's or performance's name. Or, search eBay under "Entertainment Memorabilia." Then select type of theater props you are looking for.
Visit Premiere Props and search the collection. Many of the props are from recent productions. The most coveted props go first so check back often.
Contact the set designer or costume designer and get a first hand look at the props. Contact other theater professionals and ask where they sell their props. Agents are also helpful if you can find one who will talk to you.
Visit Sale on Theater Memorabilia. They have a website that is updated regularly. The props are divided into movie or theater props. The website is then further divided into clothes, nick knacks, swords and other large props.