The reason Marc is asking,"Why Wordpress." If your looking for a designer, it wouldn't matter what platform you use. The designer is just designing the design (used the word design three times is a sentence - life goal accoplished! lol). The programer/front-end developer would be the one messing with the coding and would need to proficient with WP.
Hyper6 Design for PSD designs. I believe they can also code it into Wordpress for you. Generally, it is a better option to hire a designer for a PSD design and a front-end developer to code it onto a WP install. https://www.hyper6.com/
Side note, I think designers that understand CMS's and at least some code, responsive etc will save a lot of time/money on coding, so IMO OP's original question is valid.