Hi there - themes can't be uploaded to WordPress.com. That's a feature of self-hosted blogs through WordPress.org.
However, we offer over 200 themes, both free themes and premium themes created by leading theme companies. You can view all of our themes at http://theme.wordpress.com/
You can also post suggestions for new themes we might add on our public user forum:
If you don't want to switch to a self-hosted WordPress site from WordPress.com, you might try asking for a subscription refund if you explain the mixup.
Just let us know if we can be of further help.