Joe, I have not really targeted property managment companies, but I have about 3 landlords that I clean for, the way I found them was by looking for apartments. They always ask what do I do for a living and so I tell them, I have got 5 jobs from landlords. I have cleaned 3 propeties for one 2 for another, and the landlord I just rented from said that they will be using me when they need some carpets cleaned.
One idea I guess would be to look in the claassified adds and try to give them a call and you could do like donalson suggested offer a free demo or just tell them about your buisness and ask if you can send them some information about the benefits of your buisness.
In your local yellow pages, there should be a subheading under REAL ESTATE for REAL ESTATE MANAGEMENT. Online, go to www.allpropertymanagement.com. Or Google "property management" and your city or state. My guess is you will do better with companies who specialize in managing condominium complexes rather than rental properties. The condo owners' association will usually demand a higher level of "clean" than will the owners of rental properties.