how i can contact a iPad's owner

Hello ~You have purchased a device that is activation locked. As advised by *Rudegar* the device should perhaps be turned over to local law enforcement as it may indeed be stolen. If you contact the person you bought it from perhaps they can unlock it. If not the device is useless to you. We cannot help and neither can Apple. Take a look here:

Activation Lock for iPhone, iPad, and iPod touch - Apple Support

Also:

The All Too Common SAD Reality of Buying a Used iPad/iPhone

~Katana-San~

you can't, give the ipad to your local law enforcement or a lost and found