I know that Levenger used to carry some.
A Froogle search for "plastic folder" brings up several possibilities for online retailers. Another possibility is to see if a major manufacturer like Esselte has the product you want and then special-order it from your local office supply store. Office Depot delivers my special orders to my door.
For travel, I use one of those inexpensive plastic expandable files available everywhere. Personally, I think it's easier to keep track of one expandable than a bunch of different file folders when traveling. Nothing falls out of the expandable. Get one that's easy to open and close (e.g., Velcro), not the kind with the string that has to be wound around this way and that!