Look online. Places such as Premiere Props, Prop Masters and the Prop Store of London sell authentic movie props. While these places tend to be heavy on the sci-fi and fantasy movie props, you may still be able to find props from your favorite action or romantic comedy movies.
Shop on eBay. People on eBay sell authentic props, but you have to be careful you get authentic movie props. Make sure anything you buy has a certificate or letter or authenticity and that you read all the feed-back from other sellers to assure you are getting authentic products.
Watch for certificates or letters of authenticity. All purchases of movie props should have certificates assuring you they are actually movie props and not a scam purchase. If you buy from a reputable company, they should back up their certificates promising refunds if the certificates are proven to be forgeries.
Consider purchasing movie prop replicas if the actual movie props are out of your price range. Replicas are much cheaper, but when displayed or worn, look the same as the true movie props, giving you the same decor possibilities and conversation starters.