Have you tried asking Basecamp or Dropbox who does their copywriting?
While they might not be open to telling you directly, perhaps you could ask them if they would recommend copywriters to you.
If you want to find a good copywriter, start asking around. Posting a question here is a start, but there are lots of other places you could go. Go to where copywriters hang out at, like Facebook Groups, online communities and forums, and blogs.
Where have you tried to hire your copywriters from? And do you know what exactly you'd like from a copywriter once you find one? Are you looking to hire him or her full-time? Do you have the budget for that? Or do you want help on a project-by-project basis?
If I was building a startup and looking for a copywriter, I would...
1. Figure out exactly what I'd want. This way, once I found my copywriter I could have a list of things that need to be done.
2. Figure out what I do and do not know. You mentioned you know more than most copywriters you hire. Do you know what you don't know?
3. Figure out a budget. A good copywriter will work with your budget to help you grow your business. The right copywriter sees the potential in your startup, and wants to help you grow it (so you can keep coming back to him or her for more work). Your budget is your base -- figure it out, so you can use it to attract the right copywriter.
4. Figure out what kind of copywriter you are looking for. There are several types. Direct response copywriters. SEO Copywriters. Financial Copywriters.
If you're willing to share what kind of market your startup is for, I may be able to refer you to a copywriter or two.