OmniFocus Perspective Icons
Icons & Coffee is here to help with this very problem. The group is offering a set of 50 (!) icons to be used to define Perspectives within the app. Of course, the icons can be used anywhere on your Mac, including the Finder and other apps that allow such customization.
The set is available at $4.99 for a limited time. The images are Retina-ready, so they look great on any Mac or iOS device you might have.