Browse eBay, the largest online auction house. From a resurrected corpse to dinosaur heads, theater memorabilia abounds for any amount plus shipping.
Watch for announcements from local colleges or universities holding one-day sales of theater props. Often, they'll conduct these events as fundraisers for various goodwill projects. While they can't offer props handled by the greats, institutions of higher learning historically spend money on their theater departments and you could end up with a cool prop.
Conduct an online search for phrases such as, "buy old theater props," "used theater props for sale" or "actors auctions." Auctions within the theater community are the authentic diamonds in the rough a collector seeks.
Visit the Old Globe Regional Theatre web site. They periodically hold special costume and props sales, with items used by celebrities in past performances. This is a perfect way to collect authentic theater props.
Check with online and offline prop-making groups for extras they might be able to donate or sell for a nominal fee.