I like that other investors believe that door knocking is "dangerous and desperate." It eliminates a lot of competition. Nothing beats face to face discussions. You may have to go back several times maybe with a different flyer or newsletter to gain their confidence.
Obviously don't go to war zone neighborhoods. It does take effort, all lead sources do. My goal, and that of two other investors I know here is to door knock a minimum of 15 doors a day, at least three days a week, including neighbors to learn when the owner is home. If not at home, have a brochure or flyer, in an envelope, to leave attached to the door with masking tape.
As to yellow letters, IMO their time has long passed. I've been in homes where the owner had well over 50 yellow letters all saying essentially the same thing - I want to buy your house. How would you respond to junk mail, which is essentially what a yellow letter has become?
I agree that Zillow is very inefficient and not worth much. For a list of foreclosures ( pre and auctions) Realty Trac is good. It's a subscription service but IMO very reasonable. Also, in Georgia there is an (again, IMO,) excellent teacher, Tony Youngs who specializes in teaching about the Hidden Market - foreclosures, bankruptcy, buying junior liens, etc.
Another point that most new investors don't realize is that any marketing, direct mail, door knocking, phone calling, etc. always has and always will have a very low response rate the first time. Maybe a half percent. Teachers of direct mailing all say that real success comes after 8 - 11 contacts. I mail 500 letters a month and I know investors who mail 1,000 a month, and one mails 5,000 a month. And response rate is irrelevant, it's conversion rate that gets you the house.