You will need to adopt several different strategies to identify the experts in your field and those who may have unpublished material relevant to you.
Your database search may make it clear who the key authors are, and provide their email addresses.
Join relevant mailing lists eg Jiscmail
Use social networking sites designed for academic or professional use such asLinkedIn or Researchgate.
If you are in health or social care, try CHAIN - Contact, Help, Advice and Information Network
NHS people may join NHS Networks - click on the Connect tab to find people with similar interests.