elance / guru / getafrelancer I've found some good designers at these sites and tend to stick with them.
You can allways create a contest at "buy,sell and trade => contest" section. There a a couple of good designers there. You can choose the price yourself within an acceptable range.
Take a look at Banner Maker Pro Banner Maker Pro costs $39.99 and you can make some great Banners yourself. If I can do it then it must be easy!
I use designers from freelancing websites. As what have been said, once you find a good designer, keep them.
you can try on odesk and in the contest section here in DP, Im sure you will get a high quality banners in low price Good luck!
i spent 1500 for 3d work and that project failed ugg so I ran out of money for my new project and I started to read read learn and I got pretty good at it but It can be frustrating. But if you have some cash yeh you can get some young freelancers cheep and to the graphic artist is better money then working at a second cup