WordPress skins or theme's are the basics of your website layout and can be easily changed.

But where do you find great themes to choose from?

First of all, you can start with the free themes on the the wordpress.org website and make your selection right from you wordpress dashboard under Appearance -> Themes and make your choice.

But if you really want to stand out with your WordPress website you should look at some kind of premium themes.

These are the sites I look at first:

ThemeForest for very affordable themes and with a wide range of choices and they all come with good documentation and support.

StudioPress has childthemes that use the funcionality of their Genesis Framework, so you need both if you startout with that theme, once you have Genesis, you can just use other ChildThemes if you want to switch.
If you are planning to create your own business of building WordPress website than the Pro Plus All package is the best you can buy!

ElegantThemes with great themes for a small yearly price you get access to all their themes.

WooThemes has very special theme's for you and you really need to check them before you buy anything else...