Streaming is indeed a good form of marketing, but I am afraid that their fees might be off the roof, most especially those of them with a lot of followers, but I think it's nothing a big casino or company cannot afford.
Streamers I've watched often leave a means or link through which they can be contacted in their bio , maybe try going to YouTube, try to watch some streams, find one you feel meets the standard you want, go through the streamers bio, you definitely would find a way to contact the streamer since most of don't stream for fun but as a means of earning money.
yes, that's one way of evaluating the streamers. watch few of their live streams and you can get insights on how they are doing with this activity. try checking the twitch.tv as suggested, and you will have a lot to watch. with how you can contact them, for sure, they will leave some contact details or you can send them a message if you do like how they do live stream. but remember, the more followers/subscribers they have, the higher their asking price.