Where to put CSS files

From Joomla! Documentation

The CSS files for Joomla! 1.5 templates should be located in the template's CSS folder. i.e. yourtemplate/css/template.css

You can have as many css stylesheets as you like and you can reference them either in the <head></head> section of your template's index.php file eg. <link rel="stylesheet" href="templates/<?php echo $this->template ?>/css/template.css" type="text/css" media="screen" />

Or, you can import your additional stylesheets from within your css/template.css file eg. @import url('shop.css');