* It's in a private collection: It was owned by the production company, Lucasfilm, and is now in the personal collection of George Lucas.
* Multiple suits were made: Several suits were built for the filming, and the suit you may see in photos or videos could be one of the duplicates.
You can find the original Darth Vader suit in these places:
* The George Lucas Museum of Narrative Art: Located in San Francisco, California. It houses a collection of Star Wars artifacts, including some early Vader costumes, but not the exact original suit.
* The Lucasfilm archives: Not open to the public, these archives hold a vast collection of Star Wars props and costumes. The original Vader suit is likely stored here.
While the original Darth Vader suit isn't on display, you can find replicas and other interesting Star Wars costumes in museum exhibits and special events.