If you want REO properties before they hit the market you need to work directly with the bank. Realtors are assigned REOs once the bank has decided to list the property. If there is an agent in your market that seems to list a lot of REOs then start working with them, once they have been given the property they can sell it to you before listing it on MLS although they may not. The reason they might not is because their client is the bank so their job is to get the highest price possible for the property and the best way to ensure they do that is to list in on the open market.
HUD properties and other government backed foreclosures (VA, USDA, Fannie Mae and Freddie MAC) often have a period when the properties are only offered to owner occupants before they allow investors to make a bid on the property.