YouTube channels can have a business e-mail attached to them. It’s hidden behind a button + a captcha to prevent bot spam. I would try there first.
If that doesn’t work - and you cannot tell if they have accepted your request since they are very unlikely to reply in either case - consider their social media. For one, I’m sure Twitter allows private messages.
Though I haven’t contacted them yet (my game is still WIP), the way I’ve found some smaller YouTubers is to search for smaller indie games. Usually those who come in late to review a famous indie game are also smaller and you can find them using YouTube’s own search filters.
I have not looked into Twitch streamers at all.