The American Society of Journalists and Authors allows you to post your requirements to their members via the site linked below. You will certainly find writers of a high calibre there (or one should hope so).
I've heard Elance has a good number of American writers. However, an alternate, more painful way would be to browse through a lot of finance forums, find people with blogs and if you like their way of writing, making a direct proposal for them to write for you.
Check with your contacts too. I only take freelance jobs through people I already know or have connections with in some way. I've picked up a decent amount of writing jobs through my contacts in this manner.