I couldn't find any decent high-resolution arrow icons on the web suitable for use in a UINavigationBar or UIToolbar, so I put my limited Photoshop skills to the test and created my own. There are eight icons in total: four directions in both standard and high-definition sizes. They're basically a recreation of the up and down arrows found in Mail.app and the forward and back icons found in Mobile Safari. The images are shared under a Creative Commons license, so you're more or less free to use them however you see fit.

You can grab all eight icons here. Licensed under a Creative Commons Attribution 3.0 License.