door knocking , networking, & Craigslist possibly
It might be tough to say which one is best. You have to consider cost, time, how easy to implement, return expected etc. Have you tried pay per click, cold calling or direct mail?
The list goes on and on.
for free: Using Zillow and Redfin to find crappy houses, realtors to look at MLS, door knocking, driving for dollars.
Paid leads: Listsource, tax defaults from Title company, list from courthouse of absentee owners and probates.