This is a good question for someone considering a live-action game at a convention: what about costumes?
The short answer is you do not need one.
You’re at a convention, you’re doing more today than just playing our game. If you show up in a costume from another game you just played, or a costume you’re just wearing all day, or you show up in jeans and a t-shirt, it is not a problem and game on! In fact, an unscientific polling of my memory indicates that anywhere from half to 2/3rds of our games, depending on the game, will be played by people dressed cleverly as regular convention attendees. So do not let this stop you from playing. We don’t care, and you won’t be alone.
If you do want to think about a costume, cool. We love costumes. Want to throw on a sweet hat to go with the jeans/shirt? Excellent. Want to break out a cloak? Nice. Want to completely head-to-toe it in a hand-sewn leather Renaissance hunt master outfit? Bring it.
Here are a couple nice resources for LARP costuming, including great tips for costuming on the cheap.