Microsoft stopped selling Windows XP licenses some time ago and they are getting scarcer as time passes, thus, the reason why you are having a difficult time finding a computer with XP installed. Microsoft wants the public to fully move to Vista and Windows 7.
As littlefrankus just mentioned, you can install Windows XP on a Vista only machine but you must find the drivers yourself. Keep in mind that if a machine was launched as Vista only by the manufacturer, they will not support your Windows XP installation.