Stand aside everyone I will answer this one!
When I was a child we had a dressing up box that was a sort of long wooden box with a heavy hinged lid, it was upholstered all over and probably meant to go at the foot of a double bed. It was rather like a coffin in fact and my little brother went through a phase when he was about 5 of sleeping in it on top of the dressing up stuff. Sometimes we'd notice he was missing and eventually discover him there like some kind of juvenile vampire.
Anyway, all of the dress-up stuff was grown-ups clothes thst had been discarded for some reason or other, nothing in those days was specially made and sold for kids to dress up in but there were some wonderful things like real theatrical wigs - a peroxide blond ringletty one which did for Goldilocks and Cinderella and a handsome prince/footman one with a little pony tail and black velvet ribbon at the back.
There were my mother's 70's cast-offs including a fantastic, ruined, purple, cut-velvet dress from the 1920's and a top crocheted (sp?) in silver thread, also thigh high black stockings with spangly bits in, a blue velvet cape with white fur collar...and many more items which made our dress up box the envy of every child in the neighbourhood. We did a lot of 'shows' when we were little, it's what we mostly did...
Anyway, as soon as DD1 could walk I started a dress-up box for her. It's a large wicker basket with a lid and a cloth lining which comes out for washing.
You don't need to buy special dressing up outfits, you can just put in cast-offs for example, hats, gloves, handbags, fancy slippers, things made from shiny material, long bits of net for making veils, your old nightdresses etc
Even just a whole load of pieces of material will be enough to start with.
Capes are good, fairy wings, big sunglasses.
I get a lot of things second hand tho we do have some princess and fairy dresses.
We are moving back to the UK soon and I will be shipping the entire contents of the dress-up box, it's a very important item!
How old is your DD?