I suspect you haven't found anything mostly because Photoshop is not where you'd normally create a search form. You can certainly make backgrounds, buttons and other style elements with Photoshop, but the form itself you'd create using Dreamweaver or your web development application of choice. If you're using a CMS like Wordpress or Joomla, the tools are built in.
Then you would use CSS to create the custom styling (size, position, appearance) of your form. Any of the Dreamweaver tutorials on lynda.com cover this. There's also a video by Greg Rewis on Adobe TV, from his "Taming the Web" series that covers creating and styling forms using Dreamweaver and Photoshop.