Fortunately, many computers vendor and retailers will be more than happy to sell you through their Internet stores. The biggest of these is Dell, which currently offers three notebooks and a netbook with Ubuntu Linux. HP, which is showing renewed interest in Linux for its tablets and mobile devices will install RHEL (Red Hat Enterprise Linux) on some workstations and Novell's SLED (SuSE Linux Enterprise Desktop) on some workstations, desktops, and notebooks. Dell is much friendlier to new Linux users than HP is.
Dell was, and remains, desktop Linux's biggest support among the major PC vendors.
You don't need to go to a big vendor though to get a great Linux PC. The two most important of these minor league Linux PC vendors are system76, which specializes in Ubuntu, and ZaReason. ZaReason offers a wider variety of hardware and Linux distribution choices. Besides Ubuntu, they also offer pre-installed Debian, Fedora and Mint. System76, on the other hand, has a bit more experience. You won't go wrong though going with either of these vendors.
Regardless of whether you buy a Linux-equipped PC or you opt to install it, what you want isn't some 'objective best' desktop Linux for all users. No, what you want is an easy-to-learn and easy-to-use Linux.
From my experience that means there are three good beginner Linuxes. The first, as will come to no surprise to anyone who follows Linux, is Ubuntu. The latest version, 10.04, aka Lucid Lynx, is quite good for any Linux user and it's ideal for beginners. This wildly popular distribution is easy to use and isn't going to overwhelm new users with.