I've heard a lot of people lately asking for specific types of designers. They want "Android Designers" or "Blog Designers" or "OSX Designers." This is stupid
Just hire good designers.
People who understand user experience, user psychology, and interaction design are going to design great user interfaces in any medium. There is nothing special about the web that makes it more or less challenging than designing on an iPad. They are just different. And good designers know the differences, understand the limitations, and will design something great for each.
Good designers are hard to find. Every good designer I know has a job they love. They get recruited all the time. I am not saying it will be easy to find them. I'm just saying hiring someone who considers themselves an "iPhone designer" is going to end badly. Chances are you're going to need something done on the web. If your app is awesome, you'll want it on Android.
Don't hire someone who is good at replicating Apple's design aesthetic. Newsflash: That's not very hard to do.
Don't search for a "WordPress designer." Anyone who pre-selects their tool because that's the only thing they know how to use is being silly.
Search for people who love what they do, are good at it, and want to use their skills to solve your problems.