I highly recommend Bannermaker.com. Wes Wilson, the founder, is an acquaintance I made over three years ago when I ran across their Streetmedia's htmlgames.com site.
He built the Streetmedia company from scratch. They run several very smart businesses out of San Francisco.
Visit the Bannermaker.com site. They are very, very professional and can create you some top quality banners. They have an extensive portfolio of clients. I don't thinkg they're too pricey, but you'll have to inquire.