Well, you would have found it if you do a search on Google. There are more than 80 million search results for you to go through and choose from, good luck.
You will get free WP themes from www(dot)site5(dot)com and you can find wp themes at very cheap rate at fiverr(dot)com as well.
Qualitative and simple to use themes has the oQey Themes for WordPress. They sell them for $49 each. Though you may think that this is a too high price, I assure you will get well-designed, HTML5 compatible themes and the oQey team's support every time you will need it.