Editing Themes via Photoshop

Forgive me if this contribution to the forum reeks of newb, but alas, that is what I am, and after a couple days of lurking I have not found the answers I seek.

I'm approaching my site with the plan to edit an existing freeware theme: Step one, mock up in Photoshop. Step two, export graphical bits and parts. Step three, import new graphical bits and parts into theme. Step four, poof! Website. With Firebug, this should be easy enough to accomplish, right? WRONG!

Because in order to do my plan, I need either A: A Photoshop mockup of the original theme or B: A live demo of the theme, both of which seem to be mysteriously lacking from the drupal.org theme pages. Am I missing something? How can the two very things I need be so impossible to find?

Is there a place where I can find theme downloads alongside their Photoshop mockups or a nice live demo? ThemeGarden doesn't seem to have any of the themes I like or need, and the only Photoshop mockup I have ever found was for Acquia Slate.

Thanks! I appreciate your input, and I'm at my wit's end.