OS-Commerce is a good option if you want to add a full-featured store to your site (or turn your site into a store). But it will require a database, PHP support, and a little technical know-how to get it up and running. It may be overkill for what you need. The url is http://www.oscommerce.com/

If all you want to do is add some sort of "buy" or "pay" button to your existing site, I would sign up for a paypal account at http://www.paypal.com. It has many features to easily add that type of thing to your site, no matter what you're setup is.