Costumes
There are several choices regarding costumes for your party. You could ask each guest to dress as their favourite character or you could provide costumes when they arrive. Providing costumes could be as simple as having stick-on lightening scars for each guest, or as detailed as providing every child a full costume. Unless you have a huge budget or lots of time and creativity, a good compromise is some simple accessories.You can make them so the guests are all the same or assemble a dress-up box where the kids can select their own.After rummaging through your own closets, you could try thrift stores and dollar stores to assemble a mix of costumes.
Some ideas:
Students: grey/black trousers/skirts and sweaters, black robes (housecoats, old graduation gowns),black ties (can add strips of coloured material or paper to get the right colour tie), white shirts.
Hagrid: trench coats and rough-looking trousers, and wild wigs and beards
Professors: old graduation gowns, black trousers or long dresses/skirts
House elves: old sheets for a simple bag dress, cardboard ears
Golden Snitch: paper mache a large beach ball, paint gold and wear it, along with a gold-painted t-shirt and white net for wings.
Dementor: wear all black, including a hood, and then add black crepe paper streamers
Dumbledore: old robe, long white beard and wig
Accessories
Old black-frame glasses may be lucky enough to be found, or can make them out of cardboard.Stuffed animals : owls, rats, cats
Scarves: again, could look for plain black and add your own colours. These are also pretty easy to make from scratch using strips of colours.
Wands: could have some ready-made but I’d actually have it as an activity where they can make and decorate their own (see home page for ideas).
Flying Brooms: paint handles dark brown/black and can add the names eg Nimbus 2000. As with the wands, could have an activity staion for making their own.
Badges: download patterns from the web to trace and colour.