If you're showing them designs, sometimes it helps to lie and say that you weren't the one who made it, so other people don't feel bad about criticizing it.

You can post an anonymous survey on Reddit or specific Facebook groups, and have people leave their email if they are open to a follow-up interview.

If you already have a prototype, there are paid online services like UserTesting (https://www.usertesting.com/) where you can specify tasks for users to complete and get verbal feedback.