If you are a DFIR service provider, the Sub Contractor Listserv is an email list where you can find subcontractors for DFIR work, worldwide. Simply send an email to the list with the details of your request.
Access and use of the list is free.
Hiring a subcontractor requires you to do your own due diligence research. Suggestions include online research (take a look at the subcontractor's website), CV review, and telephone/email interviews. DFIR.training provides for the connections, but not validations.