There are many third-party sites offering dummy data, but the safest and most reliable source is the WooCommerce repository itself. The developers of WooCommerce maintain an official sample dataset that is guaranteed to be compatible with the latest version of the plugin.
You cannot list variations on the same row. The sample file will often include a separate column like meta:attribute_pa_color . For complex variations, consider using a plugin, but the sample file teaches you the base structure. woocommerce sample product csv file download