I bought an OEM version of Windows XP on Newegg.com - but that was a few years ago. I have no idea where you can purchase Windows XP now - Maybe ebay?
I use vmware fusion and I prefer it over Parellels and Bootcamp.
What are you running on the Winders side?
Also, Windows 7 runs pretty smooth on VMWare Fusion for me. I think Windows 7 is a big improvement over Vista. Lots of buzz around it right now.