One of the major packaged Linux distributions from SuSE (http://www.suse.com), produced in Germany. As of release 7.1 they offer a very nice installation program (yast2), great administration GUIs, online updates and more. It comes in 2 distributions, a Personal (3 CDs) and a Professional (6 CDs and 1 DVD). There is more server-type software in the Professional version. You should find SuSE at most computer software stores, and even some book stores (like SoftPro). Of course, you can always order it online from the SuSE website or one of the major outlets (http://www.amazon.com, http://www.outpost.com, ...). The price should be ~$30 for Personal, and ~$60 for the Professional.
Unlike many other Linux distributions, it is not possible to produce and sell cheaper versions of the SuSE CDs, due to license restrictions on the installation tools. They may be copied at no charge, however.