LeoTheme provides templates for Prestashop and Joomla. These theme are for Prestashop are excellent and the Joomla ones are probably just as good. There are subscription options to allow access to all the templates for either Joomla or Prestashop but you have the option of downloading templates individually with lifetime updates. LeoTheme also makes free templates for Joomla and Prestashop.

LeoTheme also has a blog which covers aspects of ecommerce and updates on his software.