First, try a good web search with the right Boolean connectors and keywords. (E.g.: ( “Transportation” AND “California” AND “millennials” ).
Search Twitter. The most accessible sources often have active Twitter accounts. You’d be surprised who you can reach, and they’re often accessible.
ProQuest’s databases (accessible through the online campus libraries) allows you to find academic articles by professors and experts in whatever field you need.
Ask your sources to recommend other sources. If your source doesn’t think he or she can be a source, ask her or him if she or he knows someone who might be helpful.
ABS. Always be sourcing. Everyone and anyone can be a source. If you don’t have a spreadsheet or file or database that lists any and every source you’ve ever come in contact with, create one right now. Collect as many names as possible. Whatever you do and wherever you go, your hunger for sources and your willingness to catalog them will help you at crunch time.