Are Costumes Required?
Costumes, props, and other decorations are completely optional, and are by no means required for a successful murder mystery party.
However, costumes and other paraphernalia can go a long way towards making your event a fun and memorable experience. Using decorations, props, and costumes that match the theme of of the game helps to establish an appropriate atmosphere and can really set the mood for your party.
And besides, dressing up is fun!
So where can you and your guest get your costumes? Here are some possibilities:
You can rent costumes from a local costume store.
Many toy stores and mass market retailers carry costumes, particularly around Halloween. These are usually inexpensive.
You can purchase a costume from an online retailer (just do a web search for "costumes").
Each participant can make their own costume from clothes and accessories they have at home.
If the theme of the party is known, but characters have not been pre-assigned, everyone can bring old clothes, hats, jewelry, etc. from home, and create a grab-bag at the party, where players can dress according to the characters they've chosen for themselves.
Regardless of whether you choose to use costumes or not, you should really consider getting into the spirit of the party by adopting accents, mannerisms, and attitudes that match your characters — This is really the most important thing you can do to make your party come alive!