If you find an agent who will work with you, and who uses electronic contracts, you can easily churn out dozens of offers. Low likelihood of acceptance, but its a potential source of a deal.
One wholesaler in our area who actually consistently has properties gets quite a few of his deals from HUD. Those are all on the MLS, and all go through the bid process. I've had another guy tell me he was also having some success with that.
Another wholesaler who seems to get a fair number of deals is the local Homevestors franchisee. They advertise a LOT.
I see a lot of bandit signs. Don't know if they work or not, but I do see them.
Direct mail is a long term proposition. Plan on sending multiple pieces to the same prospects.
Out of area owners, and non-owner occupied properties are candidates.
Code violations and fire damage are candidates.
Calling for rent signs can work both ways. You might find the disgruntled landlord who's interested in selling or you might find a potential buyer.
I believe wholesaling is MUCH more difficult than the gurus lead you to believe. I've not even tried because I think its just a JOB same as (and a lot like) selling used cars. Make a deal, you get paid. No deals, you don't. I have a regular job that pays me pretty well and that I'm pretty good at. I don't really want to take on another one that requires a lot of people interaction (not my thing) and that I don't know very well.