There are literally hundreds of agencies and independent contractors that offer social media management and advertising management for companies.
You'll want to do a little definition of what you're looking for:
- BUDGET: many agencies charge a flat monthly retainer plus a percentage of your spend
- OBJECTIVE: what do you want to accomplish? Different firms have different experience when it comes to online objectives. Some are great at building followers, others are great at conversion, some are great at building the creative. What are you looking for?
- PLATFORM: Not ever advertiser is going to be good at every platform. And don't let them tell you that you need to be everywhere. You need to be where your ideal customer is spending their time. If you want to target Facebook, for example, do a Google search for "Facebook advertising company" and look at their reviews, presence on other platforms, and their services. Look at the kind of content they are publishing on their blog to see if you feel they are knowledgable and can help you hit your goal
I'm happy to help more, and give more specific advice, if you feel like sharing more about your objectives.
All the best,