WordPress provides free & premium themes for use with WordPress hosted websites:
There are limited options with free themes, but this can be upgraded later.
You can see all WordPress themes here: http://theme.wordpress.com/
Please Note: Premium themes have more features than free themes and will cost between £40 & £57+
Themes for use with self-hosted websites:
For high quality feature-rich themes there are many providers with 1000's of themes available.
We use a number of trusted sources some of which are listed below.