Unless you have access to commercial databases that contain SSNs, you best bet is to hire a "skip tracer" or a private investigator. 

Indian Mountain Civil Lake Association: Very Strict Rules - Each Short-term rental must be registered with the Civic Association office and an administrative fee must be paid.

I am trying to find building owners but that info is not shared by the agencies and cannot find a reliable database to gain that info.

Another way is to build a database and systematically call, text, email, and even handwrite letters to these owners.

My only thing is that in 18 months I plan to have built my database  up by 3 times what it is at now and my team keeps the leads that I add by then. 

It's a numbers game, use all the money to get leads and on the tools to help you get leads (dialers, driving app, database, flyers, posters, ads, cold-callers, door hangers, door knockers etc.)Good luck.

Also, it's always worth calling or visiting a zoning administrator to ask your questions. 

“Property owners who wish to appeal a fireworks related citation can do so by completing the Administrative Hearing Request Form and submitting the form to the appropriate City Division based on the hearing type.”https://www.hayward-ca.gov/fire-department/office-fire-marshal/all-fireworks-are-illegal-city-hayward-and-fairview-fire-protection-districts?

I will keep a database of these contacts and reach out to chat when I have an investor looking in your market.

Don't rely on what the assessor's database says.