How to locate off market multi-family around you

[Giora Sela]

I’m trying to do a general search for small to medium size (~10 doors) multi unit properties in my city (Wilmington, NC) that are not listed or even offered for sale at the moment so I can just do some research before making any moves in the future. 

Tried to look for rent ads on Craigslist but all I can find are either huge apartment complex or duplexes. 

I was wondering if anyone had a good idea on how I can find properties in my criteria any other way?