How do you find numbers or email addresses for off market Deals?

[William Bucklin]

I've recently found vacant homes that I would be interested in buying. Using either the OnX app or city tax maps I'm able to find out the owner and their mailing address. However I haven't been able to find a good way find numbers or email addresses for the owners. Or its an out of state LLC who doesn't have any names listed. I haven't received a phone book for years so that hasn't been an option. Have any of you found an effective way to obtain this additional contact info.