Where can you purchase the melancholy of haruhi suzumiya light novels?

Actually they may be right in front of you. If you go to the section at your local book store they should be next to the manga version. I go to the book store at my mall and they have different editions of the light novel. You can also check beforehand on the internet at Borders.com or Barnes and Nobles' website to see if they carry it. At some book stores they can special order it for you if you ask. If worst comes to worst just look on Amazon.com I highly recommend them.