Where To Find Experienced React Developers?

There are loads of amazing React developers, but since the demand is high, they can command heavy compensations. So the main question is: how much can you pay (in cash or equity)?

In London, a good senior React dev would cost you something like £120K to £160K per year. It is probably cheaper in Germany. The most important thing is to be clear about what you are offering. People don't want to waste time.

The first place to look is through your own network. It's more trustworthy, and you can get people to vouch for you.
If that fails, you can go through job boards, though I haven't seen this working particularly well. You might also go through recruitment agencies. They will charge a fee, and again it's negotiable, dependent on how strong your position is, and how good they are. I've seen fees from 5% to 20%.

Now, looking at the specs, I don't think it's clear what is on offer for the person joining. Are they coming in with equity or with a salary? Then how much?

You could try : https://stackoverflow.com/talent/en

By the way where have you looked so far?

The market reality is that a good full stack React developer with remote experience can earn $300k annually by working for a big but boring company.

So you either need to more clearly point out the monetary benefits of your offer, or you need to have an inspiring paragraph about how you're improving the world so that someone would want to join you for idealistic rather than monetary reasons.