I've had a similar experience with my own brand and also helping other brands with the same dilemma. Two things off the top of my head that have worked very well are:

1. Build up your social media platforms. If you do this yourself, there will be no cost - just your time but you will be on your way to building advocates for your product. When I launched my company, I had to curate my audience on Facebook, Twitter and Instagram. This was time well spent and we gained great insights and sales.

2. Identify other companies that might benefit from a marketing collaboration. For example, with my start-up we too spent the majority of our funds on the product but then approached like-minded brands to work with. The result was that by using their network of customers and marketing initiatives I was able to promote my brand and gain direct sales. Which brands did I align with? Quiksilver, Oakley and Audi of America were just the first 3. I didn't spend a dollar and they invited my brand to communicate to their customers. And yes, i still work with them to this day, my relationship has only become stronger and they now include me in some of their efforts throughout the year.

If you're interested in learning more about how to market a brand with a zero budget, I'd be happy to join you for a quick call to help identify some potential businesses you might be able to collaborate and expose your product to.