Many casinos offer free spins... some give free spins upon registration, and some have Telegram/Twitter promotions where they give out free spins for comments/reposts or completing some small tasks. If you want to stay up to date, you must follow all of them on all social networks and be active.

I get free spins from several casinos via email, mostly from the ones where I play often. Vave, RocketPot, Weiss, and WSM Casino are sharing free spins with me more than others... once in a month or two, I get 10-20 free spins with min bet. Weiss also has a lot of simple tasks (like taking a screenshot & sharing a picture of a slot) in their telegram app, as far as I've noticed sometimes free spins are guaranteed if you complete the task, which is good thing. Promotions like this are mostly full of spammers and bots and it's hard to get anything.