try out threadless.com though m not sure if u'll get a designer personally. but m sure u'll get enough designs from
www.deviantart.com, check out the various user uploads and contact someone who you think would be able to do your job. they may ask you some amount but it wud be quiet rewarding even if u spend a few. n if u r lucky, u may find designers who do such stuffs with pro quality but jst for fun, they won't charge u any!!