I'm answering this from both a founder and a product UX design perspective. It would be helpful to have an idea as to what you're actually building to answer this question properly as many times, people think they need something when they don't necessarily need it.

You're saying you need a *top* UX designer, which would imply you're building something totally new, something where there is no known best practice for the UX of this kind of solution. Assuming that's the case, then you should be able to get a top UX designer to want to work on it based on that very challenge alone. If you explain it in this way to at least one skilled UX person, then the challenge alone should make them want to share it with their network, even if that person is not interested or unavailable or has requirements outside of what you're capable of offering.

Assuming you're not inventing something totally new, I would go onto a site like Behance or LinkedIn or Dribbble and speak with people who understand UX enough to point you in the direction of best practices for that type of solution, and then I would find perhaps a more affordable UX designer and work from there with your budget or as an equity-split/hybrid comp. If the person is not a *top* UX designer/developer, then they're likely more willing to get involved in a risk/reward scenario.

Figure out if you really need top, because that's difficult if the challenge is not there. Someone skilled can get you very far along and will be more open to working with you in an affordable way.