These days, there is only a small chance that you might find a Seven costume, and it will probably cost well over $10,000 for the jumpsuits. Perhaps one of the other styles of costume Ms. Ryan wore would be more reasonable. It would be offered through one of the major entertainment memorabilia auctions (see list below). People who bought the items originally do from time to time decide they want to either get their money back out of their collections, or have changed their interests, and off to the auction house the famous item goes.
Background: The Paramount Trek warehouses, containing all the remaining Trek items going back to the Original Series, were sold starting with the large Star Trek auction by Christies 3 years ago. This was followed by several years of sales on ebay by ItsAWrap, which sale has now also ended. There were occasional sales of Trek items before this too; stars may have been given a costume by the studio, and then auctioned it off for charity, or someone who worked on the sets may have one way or another obtained an item and resold it. But the bulk of the Trek props and costumes were sold through Christies and ItsAWrap.
You can see what has been sold in all the auctions, along with pictures, at this website:
link
You don't have to make a username to do a search. Note that the search on this site requires a plus sign if you want to say first word and second word, i.e., for seven of nine, enter "seven +of +nine". Its a bit tricky to search this archive because not all titles and descriptions contain such a clear label as the above. You may also wish to search for "jeri +ryan" and include descriptions in the search, for example.
Thousands of Trek items have been sold at this point, so its a sure thing that over time they will resurface.
You can also watch ebay, in the entertainment memorabilia section, in the 'originals' categories for props, and for costumes, with keyword 'trek'. However, be advised that there is a *big* problem with fakes, and occasionally also with stunt double costumes represented as the one worn by the star. Look for sales that say they can provide the "COA" (Certificate of Authenticity) from either one of the auction houses below, or ItsAWrap. Lots of fakers will tell you there's a "COA" but its from them, an unknown seller. This is often worthless and in fact a huge red flag. If you buy an expensive prop/costume and you can't prove its history all the way back to the studio, you run a big risk that no one will buy it from you in the future, not to mention you may have bought a fake.
Auction houses to watch are: Christies, Bonhams, Propworx, Profiles in History, Heritage Auction Galleries, Premier Props, and Superauctions.
