Haley Productions sells murder mystery games for large groups of 20 or more: http://www.murder-mystery-game.com. Be sure to read their testimonials (they have quite a few pages of them). They have about 13 or 14 themes to choose from and you download your game right away. No waiting, no shipping and handling fees. I can tell you from experience that doing a murder mystery game for a large group of people is lots of fun and absolutely, positively doable.
I took the liberty of cutting and pasting from their website how their games work:
Depending on which mystery game you select, you'll need to provide 6-8 people to play the primary suspects. Most of Haley Productions' mysteries provide you with 2 versions of the mystery: The Know It All Version (where the suspects will know whodunit) and the Narrator Version (where you'll add a narrator to narrate the mystery while the prime suspects act it out. They will not need to know whodunit in this version).
The rest of your guests will take on the role of detective and it will be their "job" to actively solve the case by examining evidence, finding clues, following and questioning suspects - whatever it takes for them to solve the mystery. The invitation included with the kit encourages them to come in costume and in character. They have the freedom to do so or just come as themselves to enjoy the mystery. No pressure to take a role or do something that takes them out of their comfort zone.
Sorry, I played tons of pen and paper but never played live action RPG before. I once saw in convention that players are running around in the balcony to play some battle game. That was probably a live action game. I am in LA, CA, US. Do you know any live action game location here? I would like to give it a try.