Have you already contacted them at all?

the first thing I would do is just contact them and let them know you love their work. What designer doesn't love to hear that. See if you can get them interested in your project at all. That has to be number two on your list. It's better to have a good designer who is in love with your product than a great designer who is only half engaged.

See how the conversations go, and if they are interested at all. You should get a feel for it. If you can't engage them directly, maybe they have other contacts they would recommend for you. I'm assuming you're in an area where designers are in high demand, so realize you'll likely have a difficult time getting anybody, let alone your top picks.