My lovely sister turned me on to these expanding files at Target. I've seen different ones but none that I like better than these. I like these because they have 13 pockets and a zipper file and I especially like the way that they close (securely). I bought different colors to help me organize and spot the "color" that I need easily.
They sell for $3.99. The expanding file is about 5x7" and fits nicely in any purse/bag. It also comes with little stick on tags so you can label your pockets.