The bootstrap grid system is so popular that people have created free bootstrap themes that you can use for your site. Notwithstanding, you should always check the license of your theme before you use it.
One bootstrap themes resource to start with is free bootstrap themes.
For this resource above, you can first preview the theme. If you like it, then click the download button for a link to a minified version of the code. Simply copy the code and paste it into a new bootstrap.css file, if you wish to use it on your site.
Another one you might consider is commercial bootstrap themes.