Sources of participants for user research and feedback

EDIT: The products we design are B2C and used by regular folks like you and me. Think Skyscanner, Zalando, etc

I'm re-evaluating the sources of getting participants our UX team uses to recruit participants. These participants will be mainly invited to test prototypes and conduct interviews.

Besides the following, I'm curious if there are any other potential sources.

• End users who are current customers. they joined by signing up on the user engagement program.
• New employees, since they are still somewhat unbiased
• Employees from other departments since they are not too close to the product team