Kim,
My knowledge of it is that people who hire post construction cleaning services are small and mid size contractors without their own cleaning service in their employ. I know of two ways to break into this industry. One - Drive around much like other starting subs and look for new construction, walk up to the site, and ask who the boss is, introduce yourself, and get the job. Second, (this one is more profitable I think) get hooked up with building managers in large strip malls or office buildings. Construction is constantly going on there as new tenants move in and out and walls are being moved around etc. This work will also be less dangerous to clean up. I would find the book that your local home builders association publishes and call every single builder in the book. Good Luck!