Who you use depends on what you are marketing and how much you are willing to pay for it.
Look up your local respected marketing company - they will have the know how to help you with a content marketing campaign. If you are looking for a more wallet friendly solution, look to virtual workplaces like oDesk.com to find freelancers with proven experience.
There are many marketing professionals who moonlight on such sites to make extra cash. There are also specialists in certain types of content marketing so look for one who specializes in the type of content you provide.