If you have access to a chem lab this video will show you how to make your own:

http://www.youtube.com/watch?v=dfNByi-rrO4

If you'd rather not make your own, you can purchase a pen full of that ink on their kickstarter page for $20 here (11 days left):

http://www.kickstarter.com/projects/electroninks/circuit-scr...