There are many affordable designers out there who can design inexpensive, high quality banners for your promotional campaigns. Research your designer carefully before spending any money. Ask for references, look at samples of their work. Don't make the deal by e-mail, talk to your designer, in person if possible. Make sure they are someone you can communicate with.
Lack of communication is the most common cause of customer dissatisfaction in the service industry.
As with most things, a little time and effort spent up front will yield much better results in the end. The next issue related to banners is the hardest to swallow. It is a sad fact, but advertising banners have a very short shelf life. On average, banners will start to lose their drawing power after about thirty days. A truly exceptional banner may live as long as ninety days.