Use a DigiCam
My Digital camera has very good stereo audio and a setting for recording just audio without pics or video. Makes good WAV files to download into the computer OR just as easily I can play/record them directly from the camera's stereo audio out into my multitrack recorder. I've done quite a few excellent outdoor recordings this way.
Go to a ball game, a concert and capture a bunch of various clips. A school or local theatre play (even if you don't like the play) can be an excellent source for applause, snickers, guffaws, a wide variety of crowd reactions. Capture it all and file it for later use. If you want a 'bigger' crowd then mix multiple tracks and process them a bit. Alternatively get a bunch of friends to augment the crowd sounds with whistles, catcalls, whatever and mix those in as well.