mojoPortal content management system ships with around 30 skins each of which could be used as a starting point for making your own custom skin. If you are upgrading, note that new skins will be under /Data/skins, if you want to use the newer skins in your site you need to copy them from /Data/skins to /Data/Sites/[SiteID]/skins
There are also some extra skins available as a separate download, extraskins.zip from our Codeplex download page.
You can also create designs using Artisteer.
Artisteer makes a few free designs available on their site. You can also find some nice free Artisteer templates on FreeArtisteerTemplates.com
You could also also hire a designer from our consulting list to make a custom skin.