Just did a quick search in google and found this https://streamsentials.com/casino-streamers/
Based on that list, these streamers has many followers, the top one has 1.7M followers so it can be a great marketing tool if a casino hire or get a deal with him.
How to contact them? I think most of them has their own social media account so you can contact them through private message in the social media.
Or you can contact them directly from where they are streaming such as twitch or youtube.