Flash paper is paper that gets treated with some very powerful chemicals that change the molecular structure of the paper.
A site with some tips on how to make it is here >>>
http://www.magicbunny.net/tricks/341.php
I don't know how easy these chemicals will be to get hold of given todays terrorist fearing climate but you will need sulphuric acid and nitric acid. Read the above article, I've never tried but if you do be sure to understand the risks of the compounds you are working with as any mistakes could potentially be dangerous. But if you get right the results will be good.
One question though, how much does it cost to make it as opposed to buy it?