In my experience it is the asset manager for the corp/bank/lender that assigns the contractors. On occasion the total responsibility could be assigned to an agent or broker for contracting. Usually though it is after they get the listing from the bank/entity. Contact the broker or bank/corp that owns the property to get on their list.
Best place to check with are asset companies that handle foreclosures such as Atlas, nationstar, and others. You can google for further names. Also, register with Fannie Mae as a service provider. Good Luck!
Look on craigslist and other websites and search for that term or for sub contract work. Required lic. tools, generator, air compessor, ability to winterize plus basic contruction, etc. One company I know of is Simplicity Services LLC. My husband worked for them for awhile. They are based out of a different state, but have bank contracts in different areas around. Might be in your area, you will have to check. Good Luck!
Takes a lot of legwork to get in the door, but you can get a lot of business by doing the right thing. You should get dressed reasonably, have brochures in hand, and hit the bricks. Visit as many REO listing agents as you can and speak to the lead broker or agent for the REO team you're visiting.