Generally I think this is unlikely to happen because people who are freelancing often are doing so because their need is liquidity (read: cash, relatively soon) and not equity in a maybe successful venture. Thus they're looking to be paid now for work, not be involved in a longer term personal investment, whether in someone else's company or their own. So by definition, someone freelancing has very different goals than someone with the security and desire to be thinking about startups. For your question, I think you'd need to find someone who very specifically is freelancing but also is looking for a good startup idea, which I think is rare.

As an example, here is a freelance job offer. The project award is less than $500, but it sparks an idea for a niche app service for photographers. https://www.elance.com/j/custom-map-flyover-tour-of-route-ar...

I turned my one off freelancing job into a startup. But my startup is a mobile development company, so I'm pretty sure I didn't answer your question.

From what I've seen, I haven't seen many people turn a freelancing gig into a product.

It happens all the time, but it is critical to be up-front with your initial customer. The good thing about it is you start with at least a bit of market validation (i.e. someone willing to pay for a solution).

True. And it may not be the exact product or service, but the project opens my eyes to a larger market opportunity.

That's the thing: what's a freelancer going to productize if all of their work is work-for-hire?