I was wondering if anyone knew of any css style repositories that you could use with StyleCatcher other than the default styles 6a provides.

StyleCatcher is so new not many of the style sites have tested and gotten compatible. Mine will be in a couple of weeks (I hope).

Probably the quickest and easiest way is to edit the existing ones.

Actually, I think very few developers will create anything to work along the line of the default styles and templates.
My 2 cents is throw them out, and use styles and templates that keep the basic priniples of usability and simplicity.
Follow the rest of the world using Web Standards which includes style recommendations.