Hi there, on WordPress.com you can choose from 200 gorgeous themes, which you can review here: http://theme.wordpress.com or try out under Appearance > Themes in your Dashboard.
Third-party themes (from the likes of themetrade.com) can't be uploaded to WordPress.com. That's a feature of self-hosted blogs through WordPress.org. See here for more information: http://en.support.wordpress.com/themes/adding-new-themes/
Perhaps you'll be able to find the a theme that meets your needs in our Theme Showcase?
You may also want to look into purchasing the Custom Design Upgrade to customize one of the themes we offer: http://en.support.wordpress.com/custom-design/
If you are considering moving to WordPress.org, I recommend reading about the differences to decide which one is right for you: http://en.support.wordpress.com/com-vs-org/
If you decide to move, we have a guide here: http://en.support.wordpress.com/moving-a-blog/#moving-to-wordpress-org