Unfortunately, you can't just go down to your local Best Buy or Sam's Club and get one. While Linux PCs show up from time to time in retail outlets, the odds are against you finding one. Instead, you're going to need to order one online. The good news is that there are numerous, well-regarded vendors that carry Linux systems.
Indeed, many big name computer manufacturers, such as Dell, HP, and Lenovo will sell you, or your business anyway, Linux PCs. I'm sorry to say though that even Dell, which at one time did a good job of supporting Ubuntu Linux, has made it very hard to pre-order Linux-powered PCs. It can still be done, but it's not easy, and the truth of the matter is none of these vendors currently make it easy for an individual to buy a Linux PC.
Instead, your best move is to buy a Linux desktop or notebook from one of the smaller vendors that specialize in Linux. You may not have heard of these companies but they've all been in business for a long time and delivery quality goods. You can buy from them with confidence.
Eight Virtues prices tend to be a bit lower than other Linux-specific desktop resellers. Many of the others aim for power-users looking for high-end hardware. At Eight Virtues, you can get a basic box and then decide if you want to move up.
Emperor has been in the Linux PC business since the 90s. These days they sell high-end Dell Latitude and Lenovo ThinkPad T and W laptops. On these, you get your choice of their own house-brand Emperor Linux, which is based on Fedora; Ubuntu; Red Hat Enterprise Linux (RHEL); SUSE Linux Enterprise Desktop (SLED); openSUSE; Fedora, Debian ; and SlackWare. They also offer not just the current versions, but older ones as well. At Emperor Linux, you really can have your Linux PC your way.
If you're new to Linux, I'd recommend you go with Ubuntu 11.04 or 10.10. Emperor offers both versions.
Los Alamos also specializes in high-end Linux-powered laptops and computers. They offer Lenovo ThinkCenter, ThinkStation, and ThinkPad T and X series laptops. On these, you can get a selection of Linux desktop distributions including Fedora 15; Ubuntu 11.04 and 10.10; and the nothing but free-software Tisquel Linux distro.
Another interesting plus for Los Alamos is they sometimes offer clearance units. Since, from where I sit, you can't go wrong with a ThinkPad for your laptop, I'd keep an eye on this area if I were you. You just might be able to get a steal of a deal.
It's a toss-up between system76 or ZaReason as to which is the better known of the Linux-specific desktop companies. Both are very good and they've earned their reputations as being outstanding computer vendors.
System76 builds their own laptops and desktops. They also offer servers. On all their platforms, they only offer Ubuntu 11.04. I've used system76 laptops myself. They tend to be very solid and well-made.
ZaReason also offers laptops, desktops, and servers that they've build themselves. Like system76 they offer a range of systems from the inexpensive to the Chimera. ZaReason calls this last one the most powerful Ubuntu laptop around. With its Intel i7 quad-core processor, USB 3.0 ports, and up to 12GBs of RAM, they're probably right.
On these machines, ZaReason offers Ubuntu and some of its variations such as Kubuntu, with its KDE desktop. In addition, the Calif.-based company offers Fedora and Mint on their hardware.
So which one should you shop at? I don't have a particular favorite. While I'm prejudiced towards ThinkPads--I love the TrackPoint--I've at least played with laptops from all these vendors and I've talked to their customers. Each of them has their fans. I recommend shopping around and finding what looks like the best deal for you.
I can also guarantee one thing: When you call any of these companies with a question, you'll get answers from people who know what they're talking about. That's more than I can say for any Windows PC vendor these days. Yes, I'm looking at you Dell!