A great place to find a vacant house right BEFORE it becomes vacant is at a garage sale. When someone is going to move, many times they have a garage sale to get rid of their unwanted items. It never hurts to ask if they are looking to sell.
Estate sales are another good place to look. If you see an estate sale advertised, always write down the address. Going to the estate sale might not be the best use of your time because a lot of times the sales are run by a third-party company and heirs are not present. My suggestion is to look up the owner on record and send them a letter.