I'd probably try to find a sample at freesound.org or someplace similar. Or perhaps as an economical variant of the previously offered advice, maybe record the audio output of your TV during a sports game, and make samples of the crowd cheering. At some point you should be able to grab a snip where it sounds like they're saying "hey". It could be a long section where they're just cheering and saying all sorts of things, but if you cut it and apply a quick fade in at the beginning and a slower one at the end, it might make a convincing "hey" sound.
If it sounds more like an "Ah", I imagine you could use some drastic eq'ing to give the illusion of more of an "a" sound.
For clarity, blend in some samples of you and a friend, or multiple versions of yourself (if you're like me and have no friends) saying "hey" and tuck those deep into the crowd sample to add the "hhh" and the "yyy" sounds. I'm just thinking out loud here, but it seems like it could work if you can't find a sample and are down for some experimentaion. It could also feel pretty rewarding if it works out. If you choose this latter approach I'd like to hear what you come up with. I've made myself curious now.