Freelance direct response copywriter and marketing consultant. Earned a degree in Interpersonal and Organizational Communication, but rarely has it helped me create the millions of dollars in sales copy I've written for my clients. Here on Clarity, I can help you with creating an attractive offer, understand your market, generating Big Ideas, and more.
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.
It sounds like you're looking for a hook - not a niche. Since you already have a product, you are already serving a market. You actual product is what determines your niche.
However, your niche may change. For example, you find out that your product is desired more by a certain type of customer that you didn't think would want it. That would be a new niche for your product.
If you want to find your products hook, you got to understand the people that buy it. Without having any idea what your product is, it's hard to talk more about this with you.
But generally, think about the reasons why people would want your product. If you've made sales, ask the people why they bought it and what they'd suggest to make it better; then, use that information to put out more products.