You are most likely looking for a Copywriter or Content Editor, if the material is primarily support documentation as opposed to marketing then you may have better success with a technical writer. Normally a copywriter won't add content to your site, rather they will supply the copy and expect you to manage it. If you want someone to manage and provide copy as well as perform other activities such as managing Adverts, SEO etc then you are looking at more of a general internet marketer. Where possible I believe its best to get specialists rather then generalists.
In terms of where to find them we have had success from sites like peopleperhour.com which tends to attract a slightly different group of people then odesk or elance.
However once you find a good copywriter keep hold of them, for my own stuff I have a copywriter I have known for years and she acts both as a writer and editor for me, because she gets me and my style of writing she is able to both ghost write for me and act as my editor and it makes such a difference to know she is there.
When looking for a copywriter make sure they get your idea and get you. In effect they will be creating the tone of voice for your site, that is a tone you have to be comfortable with I can't emphasis enough that cheap is not a good way to go. If you are on a tight budget you might consider an editor as opposed to a writer to edit your own efforts.
If that's the case, I've seen some businesses have great results using https://odesk.com or https://elance.com. Elance seems to be better for what you're after. You'll really have to be careful about who you hire, and don't be afraid to go with the person that charges a little more but has a good reputation. You'll see what I mean.
You're looking for a Copy Writer to Describe Your Product, that's what I would put in the headline.