Recommend 99designs - they have an excellent customer service. When briefing in your design be very specific in your brief. It is a problem if you are not sure what you want, so think about it and then communicate what you want in your brief. Also if you don't want designers to use icons or images you have seen over and over again, metion this in block caps in your brief. Go out and look for designers and invite them to your contest (quite a few of the high level ones don't do contests but there are many others to choose from). Overall we are super happy with the end result so we recommend 99designs.