Can you start by solving a problem you already have? Or one of your clients problems?
I have yet to "strike gold", but I've found some success with validating ideas (paying customers) with the following technique..

If you are a developer, then don't start looking outwards for ideas.

Look internally, inside of the existing clientele's businesses. If you can present a compelling idea to them, they are going to me more likely to become early adopters / paying customers too. They already know + trust you.

I personally have found minor success with this tactic. When I try to "sell an idea" in any way shape or form to a person who has no idea who I am, then it's harder. Credibility is a tough hurdle to overcome if you have none.

But, my clients were already happy with solutions I have provided them in the past, and I have a deep understanding of their businesses over-time. You might be the same?

Attack their largest problem with your skills.. Not just what you were hired to do.

Work on it at home, in the mornings, and get it to a real-MVP. Something that your clients would not be ashamed to use in production or in the field. Since you have the relationship with them prior to product-izing, you should understand their wants and needs.

If you can solve their problem, they can become your first customer.
Obviously this way should come with a couple of fair-warnings: you could end up building a custom solution. That's ok-- just charge it according to the value. If you find a pain large enough for your clients, I'd be willing to bet that it's a problem that other businesses have.

You could also take the approach completely of focusing on a specific market, then identifying their current pains. Find out their active issues, how they solve them currently, and look for ways to improve. You don't have to start from scratch, or re-invent any wheels. Just take a wheel that is working "ok" for a specific market, and figure out how you can make it "fucking awesome!"

-- In theory, it sounds great. I have yet to find success this way personally :)
Addressing specific markets becomes hard without direct access to them, and a good relationship. I got "booted" out of some communities because they saw me as an outsider who didn't pay his dues.