Good advice below, especially Lori's. As you probably realize by now, the people you're calling are just "blowing you off."

You might also contact Building Service Contractors Association International at http://www.bscai.org They represent commercial cleaners and may have some advice (or even some publications) on the subject.

Here are a couple of additional possibilities:
http://bit.ly/Cleanout2
http://bit.ly/Cleanout1

Hope that helps.

I suggest looking at what some other companies are doing on the front line of the industry. There are a lot of "handyman" contractors with no insurance or license and are not bonded that are trying to solicit business to the banks. The truth is that banks are hooked up with asset management companies that contract out to contractors. They are under lots of stress so usually go with the bigger contractors that are licensed and insured... and capable of handling the loads. I suggest looking at companies like Bristol Builders in Los Angeles http://bristolestoration.com or Safeguard Properties in Ohio. http://www.safeguardproperties.com.
Both are good place to start looking.

Lori gave you a great answer. Could agree with her more.!

John, You should contact a local agent that deals mostly with REO properties. They will have all the information needed. Sometimes they can recommend you to the asset manager of certain properties. This way you can build a relationship and hopefully get your foot in the door with some of the larger companies. Good luck!