Direct from Adobe...
https://store2.adobe.com/cfusion/sto...ionmethod=FULL
It works out cheaper buying the full Adobe Creative Suite... but as Khalid said earlier in this thread its certainly not cheap!
I don't know of any open source vector based graphics programs, unlike GIMP which is the OS equivalent of Photoshop...
EDIT: Sorry just googled and found something called Inkscape...
http://www.inkscape.org/
Not sure how good it is? But might be a good place to start before investing in Illustrator.
Couple more OS alternatives listed at the bottom of this page
http://www.osalt.com/illustrator
Again, not sure on what they're like as never tried any of them.