I just know that many professional cover artists that I discovered through normal routes also participate in those sites, so I figured I'd mention it. (jamigold.com)
Most professional cover artists will work with you to get what you want, and will professionally part ways if it's not working out. (susankayequinn.com)
Professional cover artists leave at least 1/3 of the art open for the placement of the title and author's name. (lulu.com)