(The reference comes from The Thick of It, Season 4, Episode 2 and then recurring through the rest of the season. It's a BBC show but they have it on the internet in the US.)
All you need is an all black outfit, a black eye mask, and something to indicate how quiet you are.
You can also get a 3/4 Batman Dark Knight mask from Walgreens.com for $12.99. While you're on that site you can stock up on Batman band aids too.
For Quiet I like the idea of a vest made of acoustic foam. You can buy single 12" squares of Sonitex from foamorder.com for $3.90 each.
|http://www.foamorder.com/acoustic.html $3.90 ea|
I would make a vest out of black vinyl and use rubber cement to attach the foam to the front and back of the vest (be sure to change the direction front and back.) Or if you don't feel like going to all that trouble you could just pin the foam to your shirt with safety pins. I actually was getting some other stuff from foamorder.com so I had them throw two squares of Sonitex in the box for me. When it arrives I might throw together an outfit to illustrate.
I might also add some bat wings. There are some online for sale, but I would make some by sewing some black fabric cut in bat wing shapes to the sleeves of my shirt. If you can crochet it would be awesome to make bat wing arm warmers. For a guy this would probably be stupid. Some cool batman gauntlets wouldn't be amiss though.
For an added hint you might want to download a Sound Pressure Level app for your smart phone and show it to anybody that doesn't get your outfit. Then make them sit down and watch the whole episode until they get it. (This is why I don't go to parties.)
This video won't let me embed it. Big Bang Theory Halloween costumes. (from @Rojsmith) http://youtu.be/Y5KaeCZ_AaY