I find direct mail to be mostly effective with sourcing vacant land, but no luck with existing houses to rehab. There's two big institutional wholesalers in Atlanta, and they each spend upwards of $15K per month alone in just direct marketing. Very hard to compete money wise.
Hubzu, Auction.com, HudHomeStore are good options, but good deals are few and far between.
Craigslist provides best off-market opportunities, but there's a few scammers on there as well posting properties they neither own or personally have under contract. When the time comes to show and prove (provide their contract with the owner for my attorney to review for instance), they don't reply back. I've wasted so much time chasing these deals on Craigslist - scoping them with my contractors - only to come away nothing. So to prevent wasting time, I ask wholesalers to forward some form of proof to my title company that they do indeed control the property - no luck yet in this regard.
I just have to keep grinding.