The WordPress community is full of skilled designers, and there are thousands of themes available for you to choose from. There are some good premium themes. There are also some generous people that offer their themes for free, while others offer their themes under various licenses such as the Creative Commons or GPL licenses.
A good starting point when searching for themes is the WordPress site http://wordpress.org/extend/themes/. There are lots of other good sites, a few of which are listed next:
In general, WordPress themes do work with WordPress MU. However, it is worth searching for a WordPress MU-specific version of a theme, as it may have some extra WordPress MU-specific features.
Be sure to read the license file for any theme you are planning to use, and make sure you comply with the terms of the license. Usually, all that is required is a link back to the theme designer...