I was wondering the same thing, but then decided to just do color changes on the default icons to better fit the theme I'm making.
I also found a TON of icons in the plazmic program folder. Just go to C:\Program Files\Plazmic CDK 4.5\samples\Themes\Images\...and you'll find several folders packed full of icons. Just be sure to check the icon sizes of the icons you want to use, and re-size if necessary.
If you're still left wanting new icons, do what NickfromIL said. I think there may be a few icon sets rolling around this forum, as well as the *other* BB forum.