Since I have been on the designer side of your question and some of the previous comments are helpful with finding where designers are but personally might not be what you needed, hopefully I can tell you the process that worked best with me since I have designed for over 50 different brands over the years including major labels.
You can see my portfolio to verify my experience: art-hems.com/portfolio/
When looking for work, I usually stuck to Craigslist posts or industry sites likeMalakye.com, Coroflot.com or Stylecarrers.com. Mintees and the others "tee shirt designer" websites have artists that are usually creative a specific style where these other sites are more for commercial artists like me who design comfortably for most genres and styles with minimal direction. So usually once I linked up with someone of interest, I would listen to there oversll passion and then ask my own questions.
Things to help keeping artist 110% on course would be strong visual samples along with you to express your overall vision. Make sure the artist can repeat back those ideas in thier understand and confirm by asking artist to invest less than 10 minutes some quick sketch mocks. Make sure they have an online portfolio of there once and once you know they are on the same page, it is best to talk pricing.
Pricing ranges from $10 - $600 p/design depending on your source. What you DO NOT want, is someone who only designs in Photoshop because most shops charge for film seps outside of a vector format. Some with previous experience know how to prep both Photoshop and Illustrator files for film output.
FIND A DESIGNER:
- with previous screen printing experience.
- can provide film files, at size w/ PMS color info
- provides production spec sheet (art-hems.com/downloads/blank-scn-prod-template)
Spending the extra money to receive all of that will free up your time to manage other parts of your business.. On average, I stay between $100 - $350 per design and pricing is raised higher due to amount of custom hand illustration I provide in the design along with custom lettering..
Beyond Graphic Design for T-shirts, I am able to provide the full package from the Shirt graphics to the packaged shirt with print and tags on it.
Hope this was helpful and I'll look forward to hearing from you some time!