I occasionally search for design stencils and keep an eye out for them via Google Alerts and so on. Yahoo has a library of stencils available at Design Stencils - Yahoo! Design Pattern Library which has a set available in Visio, Photoshop, Illustrator and OmniGraffle format. For OmniGraffle stencils, there is also GraffleTopia.
If you are not familiar with the concept, design stencils are an illustration that is made available in a design or illustration application, so that you can just quickly drag the illustration from a picker menu, to your drawing canvas. That way, instead of reinventing the wheel and illustrating an office chair every time, you can just drag a stencil in and voilà! you’re done. Makes life easy, and makes you look good.