Amazon has these Halo 3 Covenant Energy Sword. These can inflict some serious damage if you’re a sick individual. But they are a perfect set to go with your Halo 3 Master Chief costume. I remember Link posting another one of these some time back (it might have been custom made).
edit: fixed the link.
GamesOgre posted this on the comments: Halo Reach Inflatable Sword. According to PapaStu, these were given away at Best Buys here in the U.S.