I've also subscribed to peoplefinders.com, just as @Mooro El mentioned and from time-to-time it does help me locate a number/address to those owners who seem difficult to find. It's not super accurate, but for the monthly cost I feel it's a worthwhile resource for my team to have access to.
Another thing I do is talk to the neighbors. Sometimes they have information regarding the owners whereabouts or another means of contacting them through a friend or family member.
I have yet to find a solid way to locate hard-to-find property owners except through good old fashioned hard work and persistence. Sometimes it'll take me 3+ months to locate the true owner of a vacant property but the persistence pays off. I recently closed on a house where that was the case and once I finally reached the owner, they where ecstatic that someone wanted to buy their property. Would've made my job easier if they wouldn't have went completely awol....but, that said, because they did I was able to put in the time to locate them...whereas other investors likely got burned out and gave up the lead.
Just keep searching and the deals will find you! (or the owners in this case...haha)