If you already have a quill stem (threaded headset), you need a stem with a longer quill. Look at the Nitto Technomic, for instance.

More likely, you have a threadless headset. I know of no technical reason why you shouldn't use a stem extender in that case. They're not very attractive, but that's an subjective judgment. Neither are the super hi-rise chicken neck stems, either. Either way, you can get the bars higher.