Installing an osCommerce template is not an easy task. osCommerce does not support a template installer option, as Joomla CMS or phpBB forum do, for instance. Installing a new template for osCommerce is actually a new installation of the osCommerce files. (Please, note that the installation of the template will not affect your database, which means you will not lose your data and products)
Where to find osCommerce templates?
Below you can find a couple of very popular sites for osCommerce templates. The template prices vary from $100 to $200/template.