Reaching an owner of a vacant house

[Aya Caliva]

Hi,

We have a vacant house right next to our house and am wondering what would be the best way to reach to the owner. Does anyone have the experience?

The house has been vacant for a long time even before we moved in about 4 years ago, and it looks like a mess from outside with no maintenance. I heard from one of the neighbors that the owner left to live in a different state for some reason but nobody know his number. 

I'd like to talk to the owner; to check inside if he is interested in selling or to encourage him to maintain the outside works if he is not interested. This could be a great investment if we rental and manage our current house and live in the next house and flip at a same time. 

Thanks,
Aya