Wow, this one is a doozy. There are VERY few wines like this available in the retail market today. If you live in New York try Zachys, or if Chicago, try Sam's Wine and Spirits. Though it is unlikely that they have one, they may know of where to find one. You may have to look in the auction market. Sotheby's and Christie's are the main players in this field, and you can contact them to see if there are any lots coming up. All of the port producers are of good quality, which is good, because you will not have much choice as to brand. Expect to pay thousands of dollars a bottle. It is possible that a wine of that age will be auctioned by the bottle, but most auctions sell either full cases or large lots with numerous other cases.
There are some private brokers that purchase auction lots and break them up and resell them in smaller quantities, but unfortunately I do not know much more about them than that they exist. Tracking down one of these firms may be your best (and possibly only) course.
Good luck!