Hunting down owners of vacant properties

[Will Skillman]

I'm sure this has been asked (and answered) somewhere in this forum, but... What is the best way to hunt down owners of vacant properties? I am a wholesaler and we send out 1000's of direct mail pieces and get return mail from vacant properties with no forwarding address. Struggling to figure out how to find these people. There are deals to be made with these properties. Any recommendations would be helpful...