How can I find these people? Especially people who were fired or pushed out for example?

I usually just find former employees on LinkedIn. I feel like most people are pretty honest once they're no longer part of the company (unless they're manager/execs or slightly adjacent, those types are in PR mode 25/8), and people tend to want to help others out if it's a truly shitty company they didn't enjoy.

Of course, take what they say, praise or otherwise, with a grain of salt. It's more about looking at the general theme between a few people, rather than what an individual or two says.

I didn't say it was easy hah. Meetups, Twitter, LinkedIn etc.