Other than those on Muvipix, linked by Neale, I do not recall seeing any others.
Since PrE uses Menu Sets, and there are some special requirements for them, as PrE does much of the authoring in a semi-automatic mode, they are not as easy to create, as with regular PSD Menus, such as would be used in Adobe Encore (no Menu Sets, but still PSD's w/ special Layer Sets).
If one has Photoshop, they can create Menu Sets fairly easily, but there ARE some conventions, that must be adhered to. Though I have done quite a bit of this, I still keep one of the Library Menu Sets handy for reference, so that I do not forget something necessary.
If one has Photoshop Elements, they can modify the existing Menu Sets fairly easily. The limitation with PSE is that it cannot create Layer Sets, but can edit them (unless this has changed w/ PSE 9).
If one does any modification of the Menu Sets, I strongly recommend that they work from Copies, and NOT the original Library Menu Sets. Along with studying the full layout of the Menu Sets, study the contents of the folder, from which you Copied your Menu Set. You will see several PNG files, along with the PSD's. Those are the thumbnails used to view the Menu Sets in Create Menus. Also, study the syntax of the file names. That is important too.
Good luck, and if you do find sources for additional PrE Menu Sets, please post the URL's.
Very helpful. Thanks greatly for the info and your time.