I've been using the Itoya binders like this one recently:http://www.dickblick.com/products/itoya-art-profolio-multi-ring-refillable-binders/
The list price seems a little high - you can probably do better with coupon codes or in-store sales. Still, they probably do end up a little more costly than the pre-bound portfolios. However, they have some big advantages:
1. All pages lie flat, so your print don't get curled on the edges
2. Easy to take pages in/out, so you don't have to maneuver the whole binder to insert things
3. You can put different size pages in the same binder, so you can buy a mix of refills and put them all together if you want. Refills are pretty reasonable - ~$1/page in sets of 10.
Didn't see that you wanted a zipper - so this might be a no-go. Still, maybe it will work.