Why It's Surprisingly Okay to Use REAL Uniforms in Your Star Trek Fan Film
As if it's easy to get your hands on an admiral uniform for Starfleet, you think? As it stands, though, it's not that surprising from a certain point of view. You can't do a makeshift costume and call it a "Klingon" uniform without Paramount barking at your door to stop, because truthfully you're going to need an actual "official" uniform, one endorsed by the production company -- or, at the very least, their approval to use a Halloween costume representation.
But Why? -- Why Is It Okay to Use "Official Merchandise" in Your Star Trek Fan Film?
For one thing, it protects the brand -- another thing, it promotes it. You have to understand that Paramount, Star Trek, and Starfleet from the Neutral Zone to the farthest galaxy aren't against fan fictions and film, because what makes as great publicity and marketing (fan films, etc.) is done without Paramount having to spend any money.
In other words.... They love that you guys do that sort of stuff! They just want to make sure you're using their stuff to do it. They don't want you using bootleg items or any imitations, because that can actually hurt the brand.
If you're going to revel in your Star Trek fanaticism, they're going to want you to revel in it legitimately.
Such Is the Case for Many a Star Trek Fan Film
You can see there's plenty of flexibility. But also a lot of guidelines to follow. You can learn about more of those guidelines right here. This is the basis for trademark, copyright and patent law. Rules, rules, rules (and on top of those rules, the Prime Directive sits).
Speaking of actual costumes.... How about animated ones??