Are you looking for screening services for tenants you have identified or are you looking for a company to market the property, identify and screen tenants ?
I use a full-time property management service: American Heritage Properties (renthomes.com)
However, if you do not want full-time management, you can pay a little less for them to market and place tenants in your property.
They typically charge 10% of rent for full-time mgmt and something like 5-7% for placement services. Yes they really do about 60-70% (or more) of their work during vacancies, depending on the neediness of the tenant.
I am sure other management comapnies offer similar services.
If you only want screening services after you have potential tenants, you should pay considerably less for this, but not sure if they do this service.
If this is your first rental, I would recommend using a PM service at least for screening and placing tenants, unless your situation warrants otherwise.