The age old technique is title records for tax mailing addresses that differ from the subject property. Then mail to them.
Craigslist is a more direct route. Call the owners seeking tenants. At some point, nearly every owner entertains the idea of using a management company to handle their properties.
Lastly do some poaching. Find management companies that do lesser service than you and contact their clients based on their properties for rent list.
The above mentioned simply gets you names and addresses. The marketing part of getting their attention, turning it into a conversation, then closing the deal...well, there's whole books and lots of money you can burn on that part
Success in nearly every part of the real estate business is found in consistency and persistence. Marketing for rental properties is no different. Consistency+ Persist-ency =Credible in the eyes of people who have no other way to judge your performance...those are simply the things they will expect from you if they hire you and you have already displayed them in your approach to reach them in the first place.