Whether it be for pay or for just a piece for their portfolio.
I can never find a designer for various projects.
Maybe try to post your project and ask for help on Designer News https://news.layervault.com/
Ask around in your friends. My friends usually ask me to help them out.
Also you can post up in subreddits, facebook groups and forums.
Learn it yourself, it's a valuable skill.
Find a designer that has already designed something. That's what I did for a webmail.
Ask him/her to use his/her work. behance & dribbble are full of skillful designers.