When your idea is too ambitious

You have an idea for a specific product/service in a niche industry. You have a background in said industry, and you KNOW this will work, you even know how it will work down to the details. In theory it's simple but no one has done it yet, it's just there waiting for the right person or company to do it. In fact it's only a matter of time before someone brings it to the public. But it will only be successful and profitable at its full scale. And you know it will take tens of thousands of dollars to develop, and probably millions of dollars to market.

What do you do?