Working Papers and Preprints are indexed in many different places, including:
ProQuest Central (which aggregates 48 different databases) allows you to limit to Working Papers in the Advanced Search.
OSF Preprints
SSRN (Social Science Research Network)
In addition to these databases, scholarly journals will often allow access to preprints to subscribing Libraries. These can be accessed via the Journals website. If we don't have a subscription to a specific journal's content, InterLibrary Loan can assist in acquiring preprints.