So I get the idea that knocking on doors can create deals, and I'm not afraid to do that. But how do you find the owner of a duplex or bigger when you know they don't live there?
2 ways I know of. The city will have an assessors/tax database which are usually found on their webpage. You can search by the address to see who the tax bill is sent to. The other method would be to search through the registry of deeds. Every time a property is sold, a record is made of the transaction. In my area this is done at the county level, so you would do a google search for "X county registry of deeds" and you can search by a number of different criteria, most likely address of the property.
If you see a rental listed on craigslist, zillow, yard sign, etc. You can call the number and ask them if they would entertain the idea of selling. Low probability, but you never know when you might catch someone at the right time.