UX research companies are your best bet for creating personae. They understand a lot about your users from you and your team, then go on the field and talk to a certain number of users (online or face-to-face), they will finally create rich personae for you.
If you want to conduct conceptual testing of your product or service, UX research companies will help you find right people and run formative or validation research.
What you are looking for is your marketing avatar. There is a process to develop your avatar that I go through with my clients. Before you can figure out how and where to find those people online, you must know who they are. The answer to the second part of your question can be answered after the first part is determined. Building the persona comes from someone skilled in eliciting that information from you and if available, from your marketing analytics. Once the avatar is created, then the fun begins!
Have you completed this project yet?
I am holding a 12 week course teaching students how to complete this whole process from research to execution of the Campaigns.
I'm assuming you have started already.
However, if you need someone whose core competency is creative targeting and demographic/psychographic research for the purposes of executing Online Marketing Campaigns; I am your gal!
You can use Fieldboom.com to survey your audience, which will, in turn, help you create your personas.
By analyzing various aspects of your customer base – as well as how these aspects relate to one another – you can start to get a more complete idea of who your customers are, and what they’re looking to get from engaging with your company.
You can find more info on how to segment your customer base in this blog: http://www.fieldboom.com/blog/customer-segmentation/.