I use this, and it work very well:

http://www.artprofolio.com/Art_profolio.htm

I use the 9x12 size

You can also get it at Amazon:

http://www.amazon.com/Itoya-Profolio-Storage-Display-Portfolio/dp/B0000AE68P/ref=sr_1_1?ie=UTF8&qid=1321565065&sr=8-1