The one she wears at the medal ceremony. Where did it (and the jewelry) come from? Obviously she didn’t bring it with her. Did the rebels have a spare one on hand in case she ever visited? If so, why?
The real question is where the Ewoks got a forest dress her size?
(OK, no real mystery; the fuzzballs obviously ate its previous owner)
Dress was made from an old emergency parachute (X-wing pilots do occasionally get to bail out in atmosphere). Necklace was made from polished pieces of the dome of a scrapped R2 unit.
Ehh, I’ve got nothin’. It’s dramatic license. 
One of Han’s exes left them on the Falcon.
Those buns on her head, that you thought were her hair, were actually multi-dimensional bags containing a change of clothing, makeup, jewelry, and other accessories. Perfect for the star-traveling rebel princess!
Her Royal Majesty’s matched luggage was sent ahead of her. She was on her way to Yavin in the first place.
That’s no dress! It’s a space station.
It’s a trap!
They were stashed in R2, in the compartment that used to hold the fuel for his jets.
After you sell what’s left of the Death Star for scrap, you can afford a few new clothes and some bijoux.
Yavin was where Leia had her beach house.
A droid made it up.
Otara
She was part of the rebel alliance, and a traitor; she knew where the secret base was, and when she got there people seemed to recognize her. I just figured she’d been there before, had a room in the officers barracks and had left some clothes there.
Hey c’mon! That dress gave my young self some of my first “funny feelings”. It was the gold bikini before there was a gold bikini.
Or she borrowed or bought or had one made. Where did any of them get their clothes?
They probably had a tailor droid stashed somewhere.
It’s not really clear how long after the destruction of the Death Star the medal ceremony takes place. It could be weeks later, long enough for a very important person to get, borrow or have made some formal clothes. For that matter, we don’t know for sure it’s taking place on the Yavin moon, though the architecture suggests so.
That seems like it would be a really poor decision on the part of the Rebel Alliance. The Empire knew where they were. A dozen Star Destroyers may not be a Death Star but they would really ruin the celebration party. Skedaddling would seem like the order of the day. But… instead everyone goes, takes a shower, changes clothes and has their hair done or chassis detailed. All of this could be done pretty quickly, except they rebuilt R2D2. I imagine that takes a bit more than the hour or so Leia spent in the salon.
And yet you are absolutely right. Everything about that room suggests they are still on the Yavin moon.
It’s one of Chewbacca’s spares. He loaned it to her.
They had to alter it, of course. Chewie’s too tall.
At least she didn’t inherit her mother’s clotheshorse genes. Padme would have had at least 4 dresses to
- Watch the rebels take off
- Watch the tactical screens
- Greet the few survivors
- Give out medals
Surely the one in that crew with a collection of dresses was C-3PO…