Windows XP D-Day is 6-8-08.
Monday June 30th 2008 will be the last day that Microsoft will offer Windows XP to the general public. The rumors that Microsoft would extend the date have been proven false. They will, however, continue to support Windows XP until 2014. So XP users needn't fret that you'll suddenly be victimized by hackers due to unpatched security flaws. Updates should work exactly as you're used to them currently through the Windows Update system.
Some exceptions to this are some manufacturers that are making low cost computers that are not powerful enough to run Windows Vista.
There is always eBay, where a search for Windows XP in the Software>Operating Systems category returned almost 800 results.
UPDATE: JAN 29th 2009
I guess this was an overstatement because Windows XP continues to be sold. On new notebooks, netbooks and you can find it in abundance on store shelves, online and on ebay